No error information provided when using frontend launch files

Bug report

This is not actually a bug but rather important missing functionality.

Required Info:

• Operating System: Ubuntu 18.04
• Installation type: Eloquent from binaries, launch from source
• Version or commit hash: cac43b195fb767128df1fdf494687973d538c792 (launch)

Steps to reproduce issue

Invalid launch xml file, e.g.:

<?xml version="1.0"?>
<launch>
  <node />
</launch>

Expected behavior

Some information provided about what line or tag caused a problem when loading the file. I would expect this to be provided as clean formatted output and not in a huge python traceback.

Actual behavior

A generic exception is thrown that the launch file is invalid and ros2 launch immediately terminates.

[DEBUG] [launch.launch_context]: emitting event synchronously: 'launch.events.IncludeLaunchDescription'
[DEBUG] [launch.launch_context]: emitting event synchronously: 'launch.events.IncludeLaunchDescription'
[INFO] [launch]: Default logging verbosity is set to DEBUG
[DEBUG] [launch]: processing event: '<launch.events.include_launch_description.IncludeLaunchDescription object at 0x7fb1dc627438>'
[DEBUG] [launch]: processing event: '<launch.events.include_launch_description.IncludeLaunchDescription object at 0x7fb1dc627438>' ✓ '<launch.event_handlers.on_include_launch_description.OnIncludeLaunchDescription object at 0x7fb1d2119710>'
[DEBUG] [launch]: processing event: '<launch.events.include_launch_description.IncludeLaunchDescription object at 0x7fb1cdd67588>'
[DEBUG] [launch]: processing event: '<launch.events.include_launch_description.IncludeLaunchDescription object at 0x7fb1cdd67588>' ✓ '<launch.event_handlers.on_include_launch_description.OnIncludeLaunchDescription object at 0x7fb1d2119710>'
[DEBUG] [launch]: Traceback (most recent call last):
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/launch_service.py", line 362, in run_async
await process_one_event_task
File "/usr/lib/python3.6/asyncio/coroutines.py", line 126, in send
return self.gen.send(value)
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/launch_service.py", line 273, in _process_one_event
await self.__process_event(next_event)
File "/usr/lib/python3.6/asyncio/coroutines.py", line 110, in next
return self.gen.send(None)
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/launch_service.py", line 293, in __process_event
visit_all_entities_and_collect_futures(entity, self.__context))
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/utilities/visit_all_entities_and_collect_futures_impl.py", line 45, in visit_all_entities_and_collect_futures
futures_to_return += visit_all_entities_and_collect_futures(sub_entity, context)
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/utilities/visit_all_entities_and_collect_futures_impl.py", line 45, in visit_all_entities_and_collect_futures
futures_to_return += visit_all_entities_and_collect_futures(sub_entity, context)
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/utilities/visit_all_entities_and_collect_futures_impl.py", line 38, in visit_all_entities_and_collect_futures
sub_entities = entity.visit(context)
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/action.py", line 108, in visit
return self.execute(context)
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/actions/include_launch_description.py", line 125, in execute
launch_description = self.__launch_description_source.get_launch_description(context)
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/launch_description_source.py", line 84, in get_launch_description
self._get_launch_description(self.__expanded_location)
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/launch_description_sources/any_launch_description_source.py", line 53, in _get_launch_description
return get_launch_description_from_any_launch_file(location)
File "/home/jlangsfeld/Workspaces/ros2/build/launch/launch/launch_description_sources/any_launch_file_utilities.py", line 56, in get_launch_description_from_any_launch_file
raise InvalidLaunchFileError(extension, likely_errors=exceptions)
launch.invalid_launch_file_error.InvalidLaunchFileError: The launch file may have a syntax error, or its format is unknown

[ERROR] [launch]: Caught exception in launch (see debug for traceback): The launch file may have a syntax error, or its format is unknown
[DEBUG] [launch.launch_context]: emitting event: 'launch.events.Shutdown'
[DEBUG] [launch]: processing event: '<launch.events.shutdown.Shutdown object at 0x7fb1c5be12e8>'
[DEBUG] [launch]: processing event: '<launch.events.shutdown.Shutdown object at 0x7fb1c5be12e8>' ✓ '<launch.event_handlers.on_shutdown.OnShutdown object at 0x7fb1c5bbc128>'
[DEBUG] [launch]: processing event: '<launch.events.shutdown.Shutdown object at 0x7fb1c5be12e8>' ✓ '<launch.event_handlers.on_shutdown.OnShutdown object at 0x7fb1da7f6a90>'

Additional information

The same exception is also thrown when you do have a valid XML frontend file but don't happen to have the launch_xml package installed. It's a really terrible user experience that no information is provided at all about what the problem is.

Start by reproducing the invalid XML example and a valid XML file without the launch_xml package. Read launch/launch/launch_description_sources/any_launch_file_utilities.py and launch/launch/invalid_launch_file_error.py, then trace the reported launch_description_sources path. Done means frontend errors identify the likely syntax or missing-package cause in clean output instead of only a generic message and traceback.