[MLIR] Cleanup mlir-tblgen error messages for custom assembly formats.

The messages are somewhat cryptic, since they are not complete sentences,
include lots of ambiguous words, like 'format' which are hard to parse,
and include names from the users code which may, or may not make sense in
the context of the message.  Start to clean this up and provide some
guidance for fixes.

Also, add a test for one of the messages which didn't have a test at all.

Differential Revision: https://reviews.llvm.org/D77449