enhance the warning message sent to the client

 - when a  datasource connector is referenced but is not
 installed a warning message may be sent to the client
 out of context so it can be confusing to the user if
 they are not in the context of the offending connector
 - adding a small phrase explaining that the connector
 that is not installed is referenced in one of the users
 datasource definitions

@raymondfeng 

Fixes https://github.com/strongloop/strong-studio/issues/373