None of the fields stored in the condition classes seemed like
they would be very useful.
At the call site for the undefined-field-type function it was
an absolute mystery what the error message would end up looking
like and whether the correct format arguments were being supplied.
Let's just keep it simple until there is a clear need.
Added protobuf-error as the root of our condition hierarchy. In a
separate pass I'll go through and make sure that whenever we signal
an error it is a protobuf-error.
API: Remove most of conditions.lisp
Let's just keep it simple until there is a clear need.