fsmonitor: correct an error message

Without the change, the error looks like:

  warning: Watchman unavailable: "watchman" executable not in PATH (%s),
  while executing [Errno 2] No such file or directory

With the change, it now looks like:

  warning: Watchman unavailable: "watchman" executable not in PATH
  ([Errno 2] No such file or directory)

Differential Revision: https://phab.mercurial-scm.org/D322

hgext/fsmonitor/pywatchman/__init__.py

@@ -825,7 +825,7 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()