test infra: catch and print exception info on test runner socket listener

This is the listener's spawned connection, not the listener itself.
(i.e. this is the test runner's receiving side of test event sockets).
A standard socket.error will just issue an INFO statement and continue.
Something other than a socket.error will get an ERROR: printed (and
also continue).

Hopefully this gets us more info and also handles the completely
to-be-expected scenario that the test inferior might go down at
any point.

llvm-svn: 255581