This change addresses conditions where an exception is thrown by the library upon receipt of a malformed message from an external
source, and propagated up to the event loop with no chance of
interception by the application. This issue is only experienced by
proxy convenience APIs, as low-level APIs allow the application to
unpack the message.
Strategy:
1. For malformed signals received by proxies: ignore the signal.
2. For malformed async method responses, translate the unpack
exception into an sdbus::Error, and pass it to the caller as expected.