Add information about which required field was missing

[gregor-i]

Exceptions caused by missing required fields don't mention which fields were missing, even though this information is easy to provide

[thesamet]

Can you add unit tests? They can be based on e2e/src/main/protobuf/google/protobuf/unittest.proto and use TestRequired, TestRequiredForeign, and TestRequiredOneof, it should exercise the cases were one or more than one fields are missing.

[gregor-i]

I've added some test cases. I didn't know how to construct incomplete messages, so my test data setup may be overcomplicated.