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

hgext/fsmonitor/__init__.py

@@ -193,7 +193,7 b' def debuginstall(ui, fm):'
             pycompat.bytestr(v["version"]),
         )
     except watchmanclient.Unavailable as e:
-        err = str(e)
+        err = stringutil.forcebytestr(e)
     fm.condwrite(
         err,
         b"fsmonitor-watchman-error",