fsmonitor: properly handle str ex.msg
ex.msg is always a str, since pywatchman uses str for exception messages.
This commit removes a b'' from a string compare to avoid types
mismatch and adds a coercion to bytes before stuffing the exception
message on our local exception type, which uses bytes for the message
elsewhere in this file.
Differential Revision: https://phab.mercurial-scm.org/D7855