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

hgext/fsmonitor/watchmanclient.py

@@ -105,11 +105,11 b' class client(object):'
                 )
             return self._watchmanclient.query(*watchmanargs)
         except pywatchman.CommandError as ex:
-            if b'unable to resolve root' in ex.msg:
+            if 'unable to resolve root' in ex.msg:
                 raise WatchmanNoRoot(
                     self._root, stringutil.forcebytestr(ex.msg)
                 )
-            raise Unavailable(ex.msg)
+            raise Unavailable(stringutil.forcebytestr(ex.msg))
         except pywatchman.WatchmanError as ex:
             raise Unavailable(stringutil.forcebytestr(ex))