fsmonitor: fix exception message scraping

pywatchman.CommandError formats its error message such that
'unable to resolve root' is not a prefix. This change fixes that by
instead just searching for it as a substring.

hgext/fsmonitor/watchmanclient.py

@@ -87,7 +87,7 b' class client(object):'
                     useImmutableBser=True)
             return self._watchmanclient.query(*watchmanargs)
         except pywatchman.CommandError as ex:
-            if ex.msg.startswith('unable to resolve root'):
+            if 'unable to resolve root' in ex.msg:
                 raise WatchmanNoRoot(self._root, ex.msg)
             raise Unavailable(ex.msg)
         except pywatchman.WatchmanError as ex: