Log exceptions thrown by signal callbacks

[emontnemery]

Description:

Improve logging of exceptions thrown by MQTT message callbacks as discussed in #15446

Example of improved logging:

ERROR:homeassistant.components.lock.mqtt:Exception in async_discover when dispatching 'mqtt_discovery_new_lock_mqtt': '{'name': 'Beer', 'platform': 'mqtt', 'state_topic': 'homeassistant/lock/bla/state', 'discovery_hash': ('lock', 'bla')}'
Traceback (most recent call last):
  File "/mnt/d/development/github/home-assistant_fork/homeassistant/components/lock/mqtt.py", line 63, in async_discover
    config = PLATFORM_SCHEMA(discovery_payload)
  File "/mnt/d/development/github/home-assistant_fork/lib/python3.6/site-packages/voluptuous/schema_builder.py", line 267, in __call__
    return self._compiled([], data)
  File "/mnt/d/development/github/home-assistant_fork/lib/python3.6/site-packages/voluptuous/schema_builder.py", line 589, in validate_dict
    return base_validate(path, iteritems(data), out)
  File "/mnt/d/development/github/home-assistant_fork/lib/python3.6/site-packages/voluptuous/schema_builder.py", line 427, in validate_mapping
    raise er.MultipleInvalid(errors)
voluptuous.error.MultipleInvalid: required key not provided @ data['command_topic']

Example of improved logging if not skipping the wrapper:

ERROR:homeassistant.components.lock.mqtt:Exception in async_discover when dispatching 'mqtt_discovery_new_lock_mqtt': '{'name': 'Beer', 'platform': 'mqtt', 'state_topic': 'homeassistant/lock/bla/state', 'discovery_hash': ('lock', 'bla')}'
Traceback (most recent call last):
  File "/mnt/d/development/github/home-assistant_fork/homeassistant/helpers/dispatcher.py", line 51, in wrapper
    await func(*args)
  File "/mnt/d/development/github/home-assistant_fork/homeassistant/components/lock/mqtt.py", line 63, in async_discover
    config = PLATFORM_SCHEMA(discovery_payload)
  File "/mnt/d/development/github/home-assistant_fork/lib/python3.6/site-packages/voluptuous/schema_builder.py", line 267, in __call__
    return self._compiled([], data)
  File "/mnt/d/development/github/home-assistant_fork/lib/python3.6/site-packages/voluptuous/schema_builder.py", line 589, in validate_dict
    return base_validate(path, iteritems(data), out)
  File "/mnt/d/development/github/home-assistant_fork/lib/python3.6/site-packages/voluptuous/schema_builder.py", line 427, in validate_mapping
    raise er.MultipleInvalid(errors)
voluptuous.error.MultipleInvalid: required key not provided @ data['command_topic']

Example of old error message:

ERROR:homeassistant.core:Error doing job: Task exception was never retrieved
Traceback (most recent call last):
  File "/mnt/d/development/github/home-assistant_fork/homeassistant/components/lock/mqtt.py", line 63, in async_discover
    config = PLATFORM_SCHEMA(discovery_payload)
  File "/mnt/d/development/github/home-assistant_fork/lib/python3.6/site-packages/voluptuous/schema_builder.py", line 267, in __call__
    return self._compiled([], data)
  File "/mnt/d/development/github/home-assistant_fork/lib/python3.6/site-packages/voluptuous/schema_builder.py", line 589, in validate_dict
    return base_validate(path, iteritems(data), out)
  File "/mnt/d/development/github/home-assistant_fork/lib/python3.6/site-packages/voluptuous/schema_builder.py", line 427, in validate_mapping
    raise er.MultipleInvalid(errors)
voluptuous.error.MultipleInvalid: required key not provided @ data['command_topic']

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[emontnemery]

Are tests needed for this change?

[andrewsayre]

@emontnemery this change introduces a bug when the target is a functools.partial. We need to check that the target is a partial, then inspect the contained function to know whether it should be a coroutine or not. See #20119 where this was added to async_add_job, which the dispatcher calls.

Edit: I'll submit a PR to address this -- just running local tests now before I submit.