Handle errors returned from `Conn.sendMessage`

[lthibault]

While investigating #303, @zenhack and I observed that most calls to Conn.sendMessage were ignoring the returned error. The immediately problematic call sites have been fixed, but we'll want to respond to those other errors as well. At a minimum, we might consider passing them to ErrorReporter, or terminating the connection.

@zenhack suggested it might be possible to detect this issue using a custom linter.

[zenhack]

Quoting Louis Thibault (2022-09-20 11:03:39)

@zenhack suggested it might be possible to detect this issue using a custom linter.

I strongly suspect there's an off-the-shelf one that does what we need.