Display the actual error when a pushkin cannot be created.

[clokep]

Error handling is hard. 😢

This is a follow-up to #122 to show the true error if a pushkin cannot be created, e.g. I added the following to my config:

apps:
  foobar:
    type: apns

And you get a nice(r) error:

2020-05-13 08:11:19,573 [69312] INFO  __main__ Using sqlite database
2020-05-13 08:11:19,582 [69312] INFO  __main__ Importing pushkin module: sygnal.apnspushkin
2020-05-13 08:11:19,612 [69312] INFO  __main__ Creating pushkin: ApnsPushkin
2020-05-13 08:11:19,612 [69312] ERROR __main__ Failed to load and create pushkin for kind 'apns'
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 "/matrix/sygnal/sygnal/sygnal.py", line 222, 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 "/matrix/sygnal/sygnal/sygnal.py", line 222, 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 "/matrix/sygnal/sygnal/sygnal.py", line 191, in _make_pushkins_then_start
    self.pushkins[app_id] = await self._make_pushkin(app_id, app_cfg)
  File "/matrix/sygnal/sygnal/sygnal.py", line 186, in _make_pushkin
    return await clarse.create(app_name, self, app_config)
  File "/matrix/sygnal/sygnal/notifications.py", line 129, in create
    return cls(name, sygnal, config)
  File "/matrix/sygnal/sygnal/apnspushkin.py", line 111, in __init__
    "You must provide a path to an APNs certificate, or an APNs token."
sygnal.exceptions.PushkinSetupException: You must provide a path to an APNs certificate, or an APNs token.

This will help debug #124.

[babolivier]

Should we do a 0.6.1 with this fix?

[richvdh]

are we not going to have similar problems with the other places that #122 changed?

(presumably when we raise the RuntimeError, the old exception is set as a __cause__ on the new, so an alternative approach might be to have https://github.com/matrix-org/sygnal/pull/122/files#diff-13c51378d37f89582bc66a30f28f723bR226 log the cause, somehow, which might give a slightly clearer output? not sure though)

[clokep]

are we not going to have similar problems with the other places that #122 changed?

The other places in #122 are brand new exceptions, nothing is caught in those cases, so I don't think there's anything "better" to log in those cases.

[richvdh]

The other places in #122 are brand new exceptions, nothing is caught in those cases, so I don't think there's anything "better" to log in those cases.

good point!

[clokep]

I did debate using a custom exception (StartError) and handling that separately to just print the string instead of the whole stack trace. Do you think that would make anything clearer @richvdh?

[richvdh]

Do you think that would make anything clearer @richvdh?

probably not worth it. suggest you go with what you've got.