Decode stderr in repo qrexec calls

Before this change, "foobar\n" on stderr would be rendered (in the
warning dialog) as "b'foobar\n'", which is ugly. Now it'll be rendered
just as "foobar", followed by an actual newline character.

qubesmanager/global_settings.py

@@ -47,7 +47,7 @@ def _run_qrexec_repo(service, arg=''):
     )
     if p.stderr:
         raise RuntimeError('qrexec call stderr was not empty',
-                           {'stderr': p.stderr})
+                           {'stderr': p.stderr.decode('utf-8')})
     if p.returncode != 0:
         raise RuntimeError('qrexec call exited with non-zero return code',
                            {'returncode': p.returncode})