Environment introspection failures are not handled

[mmarchetti]

If introspection of the python environment fails, a JSON object with an error field is returned. The front end should report that error and stop the deployment. Instead, it passes the (invalid) environment description to the backend which fails with this error:

[E 14:52:45.406 NotebookApp] Bundle creation failed
    Traceback (most recent call last):
      File "/rsconnect_jupyter/rsconnect_jupyter/__init__.py", line 117, in post
        bundle = make_source_bundle(model, environment, ext_resources_dir, extra_files=[])
      File "/rsconnect_jupyter/rsconnect_jupyter/bundle.py", line 117, in make_source_bundle
        manifest = make_source_manifest(nb_name, environment, 'jupyter-static')
      File "/rsconnect_jupyter/rsconnect_jupyter/bundle.py", line 20, in make_source_manifest
        package_manager = environment['package_manager']
    KeyError: 'package_manager'

[mmarchetti]

The front end error field shows Bundle creation failed: 'package_manager'

[joncfoo]

Closing in favor of #155