[split] scrooge: expose IDL annotations in generated structs

RB_ID=289353
[split] bump finagle to 6.12.3-SNAPSHOT

RB_ID=292867
TBR=true
[split] scrooge: throw an error when reading a field with the wrong type

Motivation:
Currently Scrooge silently fails when a field is specified but of the wrong type.

Modifications:
Modify readField.scala to throw an exception with more information rather than silently ignoring it.

Result:
When encountering a field with a type different than expected, Scrooge throws an exception that contains the field name as well as the expected and actual types.

RB_ID=293787
Merge branch 'tmp'