scrooge-generator: make ThriftParser strict on reserved keywords

Problem:
We want to align internal thrift with OSS standards.

Solution:
To ensure no new usages of reserved keywords arise,
we'll change the ThriftParser from optionally failing or
warning a user that they've used a keyword to always failing.

JIRA Issues: CSL-11611

Differential Revision: https://phabricator.twitter.biz/D814995

Author: joybestourous

CHANGELOG.rst

@@ -7,6 +7,13 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+Breaking API Changes
+~~~~~~~~~~~~~~~~~~~~
+* scrooge-generator: the `c.t.scrooge.frontend.ThriftParser` now always throws exceptions 
+  rather than warnings when a fieldname matches a reserved keyword. See 
+  `c.t.scrooge.frontend.ThriftKeywords` for the full list of disallowed 
+  keywords. ``PHAB_ID=D814995``
+
 22.1.0
 ------
 

scrooge-generator/src/main/scala/com/twitter/scrooge/frontend/ThriftParser.scala

@@ -109,7 +109,7 @@ class ThriftParser(
   lazy val simpleIDRegex: Regex = "[A-Za-z_][A-Za-z0-9_]*".r
   lazy val simpleID: Parser[SimpleID] = positioned(simpleIDRegex ^^ { x =>
     if (ThriftKeywords.contains(x))
-      failOrWarn(new KeywordException(x))
+      throw new KeywordException(x)
 
     SimpleID(x)
   })