Avoid negotiating version if client doesn't know the facade

This is to suppress redundant warning messages (about facade versions)
to be emitted in case the facade in question is not known to the
pylibjuju client.

Author: cderici

juju/client/connection.py

@@ -894,6 +894,7 @@ def _build_facades(self, facades_from_connection):
                 # if a facade is required but the client doesn't know about
                 # it, then log a warning.
                 log.warning(f'unexpected facade {name} received from the controller')
+                continue
 
             try:
                 # allow the ability to specify a set of facade versions, so the