Investigate Sentry error: "Could not connect to server:" from aioapns

[callahad]

Our monitoring for Syngal on matrix.org occasionally sees an error:

Could not connect to server:

For whatever reason, Sentry does log a traceback for this error; it just reports the line above.

That error message appears to come from this line in aioapns: https://github.com/Fatal1ty/aioapns/blob/v2.0.2/aioapns/connection.py#L356

So... what could cause this to happen? What calls does Sygnal make that eventually trigger that error? Can we handle the error more gracefully?

(For Element employees, this is https://sentry.matrix.org/sentry/sygnal-matrixorg/issues/15228/)