Log errors during start-up and fix the default logging config

[clokep]

This copies (a slimmed down) version of the code we use for Synapse to log errors during start-up.

With this I'm able to get a sane error message if I have a bad config:

$ python -m sygnal.sygnal
The following configuration fields are not understood: {'database'}
Error during startup:
Traceback (most recent call last):
  File "/.virtualenvs/sygnal/lib/python3.7/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/sygnal/sygnal/sygnal.py", line 218, in start
    port, bind_addresses, pushgateway_api
  File "/.virtualenvs/sygnal/lib/python3.7/site-packages/twisted/internet/defer.py", line 911, in ensureDeferred
    return _cancellableInlineCallbacks(coro)
  File "/.virtualenvs/sygnal/lib/python3.7/site-packages/twisted/internet/defer.py", line 1529, in _cancellableInlineCallbacks
    _inlineCallbacks(None, g, status)
--- <exception caught here> ---
  File "/sygnal/sygnal/sygnal.py", line 218, in start
    port, bind_addresses, pushgateway_api
  File "/.virtualenvs/sygnal/lib/python3.7/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/sygnal/sygnal/sygnal.py", line 196, in _make_pushkins_then_start
    "No app IDs are configured. Edit sygnal.yaml to define some."
builtins.RuntimeError: No app IDs are configured. Edit sygnal.yaml to define some.

Fixes #121
Fixes #120

[clokep]

This in particular had me scratch my head since...it makes a Deferred but never does anything with it.

[reivilibre]

iirc that schedules that coroutine on the Reactor (if I remember the Twisted words correctly).

[clokep]

One thing to note with this is that the process does exit with a success code (0). Adding a sys.exit(1) caused an additional exception to be raised, but still exited as success.

[babolivier]

lgtm otherwise

[clokep]

I also added the fix for #120 to this.

stale