fsmonitor: use stringutil.forcebytestr() instead of str() on an exception stable

fsmonitor: use stringutil.forcebytestr() instead of str() on an exception

Similar to 5fa8ac91190e / D7206, should get test-install.t passing on py3.

Differential Revision: https://phab.mercurial-scm.org/D7218

--- a/hgext/fsmonitor/__init__.py	Mon Nov 04 16:13:01 2019 +0100
+++ b/hgext/fsmonitor/__init__.py	Mon Nov 04 19:05:44 2019 +0000
@@ -193,7 +193,7 @@
             pycompat.bytestr(v["version"]),
         )
     except watchmanclient.Unavailable as e:
-        err = str(e)
+        err = stringutil.forcebytestr(e)
     fm.condwrite(
         err,
         b"fsmonitor-watchman-error",