Catch malformed URL import schemas

[pierotofy]

People using an invalid URL will cause:

ERROR Uncaught error! This is potentially bad. Please report it to http://github.com/OpenDroneMap/WebODM/issues: Invalid URL '5b91c372-7f4d-4438-a31d-8497233153a5': No schema supplied. Perhaps you meant http://5b91c372-7f4d-4438-a31d-8497233153a5? Traceback (most recent call last):
File "/webodm/worker/tasks.py", line 143, in process_task
task.process()
File "/webodm/app/models/task.py", line 631, in process
self.handle_import()
File "/webodm/app/models/task.py", line 578, in handle_import
download_stream = requests.get(self.import_url, stream=True, timeout=10)
File "/usr/lib/python3/dist-packages/requests/api.py", line 76, in get
return request('get', url, params=params, **kwargs)
File "/usr/lib/python3/dist-packages/requests/api.py", line 61, in request
return session.request(method=method, url=url, **kwargs)
File "/usr/lib/python3/dist-packages/requests/sessions.py", line 528, in request
prep = self.prepare_request(req)
File "/usr/lib/python3/dist-packages/requests/sessions.py", line 456, in prepare_request
p.prepare(
File "/usr/lib/python3/dist-packages/requests/models.py", line 316, in prepare
self.prepare_url(url, params)
File "/usr/lib/python3/dist-packages/requests/models.py", line 390, in prepare_url
raise MissingSchema(error)
requests.exceptions.MissingSchema: Invalid URL '5b91c372-7f4d-4438-a31d-8497233153a5': No schema supplied. Perhaps you meant http://5b91c372-7f4d-4438-a31d-8497233153a5?