fsmonitor: reapply dd35abc409ee

The recent revendoring of pywatchman undid this bug fix.
Let's reapply it.

Differential Revision: https://phab.mercurial-scm.org/D7203

hgext/fsmonitor/pywatchman/__init__.py

@@ -996,7 +996,7 b' class client(object):'
             p = subprocess.Popen(cmd, **args)
 
         except OSError as e:
-            raise WatchmanError('"watchman" executable not in PATH (%s)', e)
+            raise WatchmanError('"watchman" executable not in PATH (%s)' % e)
 
         stdout, stderr = p.communicate()
         exitcode = p.poll()