As seen in e.g. #244, #240, #205, it's relatively easy for application code to create messages that are ill-formatted semantically, and thus trigger a disconnection by the bus daemon. We should check this directly in library code to prevent one wrong message from terminating the whole connection, which would also result in better error messages:

1. Ensure that message.IsValid covers everything that already is checked in encoder.go
2. Check for valid format for error and signal names
3. Check IsValid in Emit
   Method replies are a not-so-nice case because there's no real way to give feedback from the library to the application code here when using Export (without an API break), but just writing an error to stderr (with the option to disable it) would be an alternative.