Address some confusion I encountered while testing
https://github.com/hypothesis/lms/pull/1157/ where JSON-RPC messages
from the lms app to the client would be silently dropped if the
`rpcAllowedOrigins` config was not set correctly.

Log a console warning when this happens to make the problem more
obvious.  Before warning though, filter out non JSON-RPC messages
because there are various other sources of "message" events which the
client is likely to receive and which we don't want to result in warning
spam.