setup: ignore extension load failures when finding working hg

Previously, `make local` would fail if any extension was not properly loading.

Differential Revision: https://phab.mercurial-scm.org/D2589

setup.py

@@ -255,6 +255,7 b' def filterhgerr(err):'
            if (not e.startswith(b'not trusting file')
                and not e.startswith(b'warning: Not importing')
                and not e.startswith(b'obsolete feature not enabled')
+               and not e.startswith(b'*** failed to import extension')
                and not e.startswith(b'devel-warn:'))]
     return b'\n'.join(b'  ' + e for e in err)