Add gracefully handling of undefined environment variables when parsing $(env ...) tags

[ChrisTimperley]

2021-08-16 21:54:35.493 | INFO     | roswire.ros1.launch.reader:_parse_file:150 - parsing launch file [/ros_ws/src/autorally/autorally_core/launch/hardware.machine]:
<launch>

  <group if="$(env ROSLAUNCH_SSH_UNKNOWN)">

    <machine name="autorally-master" address="$(env MASTER_HOSTNAME)" user="$(env MASTER_USER)" default="true"
      env-loader="/home/autorally/catkin_ws/devel/env.sh"/>

  </group>

  <group unless="$(env ROSLAUNCH_SSH_UNKNOWN)">
    <machine name="autorally-master" address="$(env MASTER_HOSTNAME)" default="true"/>
  </group>

  <machine name="autorally-ocs" address="$(env HOSTNAME)" />
</launch>

2021-08-16 21:54:35.493 | DEBUG    | roswire.ros1.launch.reader:wrapped:119 - parsing <group> tag
2021-08-16 21:54:35.494 | DEBUG    | roswire.ros1.launch.reader:_resolve_args:518 - resolve [$(env ROSLAUNCH_SSH_UNKNOWN)] with context: {}
2021-08-16 21:54:35.494 | DEBUG    | roswire.common.launch.substitution:_resolve_substitution_arg:43 - resolving substitution argument: $(env ROSLAUNCH_SSH_UNKNOWN)
2021-08-16 21:54:35.494 | DEBUG    | roswire.common.launch.substitution:_resolve_substitution_arg:45 - stripped delimiters: env ROSLAUNCH_SSH_UNKNOWN
2021-08-16 21:54:35.494 | DEBUG    | roswire.common.launch.substitution:_resolve_substitution_arg:47 - argument kind: env
2021-08-16 21:54:35.805 | DEBUG    | roswire.app.instance:close:157 - destroying app instance directory: /home/chris/.roswire/containers/tmprgx6ke62
2021-08-16 21:54:35.806 | DEBUG    | roswire.app.instance:close:159 - destroyed app instance directory: /home/chris/.roswire/containers/tmprgx6ke62
2021-08-16 21:54:36.119 | DEBUG    | roswire.app.instance:close:157 - destroying app instance directory: /home/chris/.roswire/containers/tmpzb2uthdn
2021-08-16 21:54:36.119 | DEBUG    | roswire.app.instance:close:159 - destroyed app instance directory: /home/chris/.roswire/containers/tmpzb2uthdn
Traceback (most recent call last):
  File "/home/chris/.local/share/virtualenvs/rosdiscover-39I1LD61/lib/python3.9/site-packages/dockerblade/shell.py", line 185, in environ
    return self._environment[var]
KeyError: 'ROSLAUNCH_SSH_UNKNOWN'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/chris/.local/share/virtualenvs/rosdiscover-39I1LD61/bin/rosdiscover", line 33, in <module>
    sys.exit(load_entry_point('rosdiscover', 'console_scripts', 'rosdiscover')())
  File "/home/chris/tools/rosdiscover/src/rosdiscover/cli.py", line 191, in main
    args.func(args)
  File "/home/chris/tools/rosdiscover/src/rosdiscover/cli.py", line 49, in launch
    summary = _launch_config(args)
  File "/home/chris/tools/rosdiscover/src/rosdiscover/cli.py", line 44, in _launch_config
    return _launch(config)
  File "/home/chris/tools/rosdiscover/src/rosdiscover/cli.py", line 38, in _launch
    interpreter.launch(fn_launch)
  File "/home/chris/tools/rosdiscover/src/rosdiscover/interpreter/interpreter.py", line 85, in launch
    config = reader.read(launch_description.filename, launch_description.get_argv())
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 546, in read
    ctx, cfg = self._load_tags(ctx, cfg, list(launch))
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 162, in _load_tags
    ctx, cfg = loader(self, ctx, cfg, tag)
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 128, in wrapped
    return loader(self, ctx, cfg, elem)
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 427, in _load_include_tag
    ctx_child, cfg = self._load_tags(ctx_child, cfg, list(launch))
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 162, in _load_tags
    ctx, cfg = loader(self, ctx, cfg, tag)
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 126, in wrapped
    if not self._ifunless_check(elem, ctx):
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 505, in _ifunless_check
    if_val = self._read_optional(elem, "if", ctx)
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 495, in _read_optional
    return self._read_required(elem, attrib, ctx)
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 501, in _read_required
    return self._resolve_args(elem.attrib[attrib], ctx)
  File "/home/chris/tools/roswire/src/roswire/ros1/launch/reader.py", line 523, in _resolve_args
    return resolver.resolve(s)
  File "/home/chris/tools/roswire/src/roswire/common/launch/substitution.py", line 238, in resolve
    s = R_ARG.sub(lambda m: self._resolve_substitution_arg(m.group(0)), s)
  File "/home/chris/tools/roswire/src/roswire/common/launch/substitution.py", line 238, in <lambda>
    s = R_ARG.sub(lambda m: self._resolve_substitution_arg(m.group(0)), s)
  File "/home/chris/tools/roswire/src/roswire/common/launch/substitution.py", line 54, in _resolve_substitution_arg
    return self._resolve_env(var)
  File "/home/chris/tools/roswire/src/roswire/common/launch/substitution.py", line 90, in _resolve_env
    return self.shell.environ(var)
  File "/home/chris/.local/share/virtualenvs/rosdiscover-39I1LD61/lib/python3.9/site-packages/dockerblade/shell.py", line 187, in environ
    raise EnvNotFoundError(var) from exc
dockerblade.exceptions.EnvNotFoundError: No environment variable found with name: ROSLAUNCH_SSH_UNKNOWN