Updated build file parser error message when the same target is redefined

Testing Done:
Added a new test in test_build_file_parser.py and updated an existing one.

I tested with

PANTS_DEV=1 ./pants tests/python/pants_test/base:build_file_parser --verbose

but am currently unable to retest or run ci because of the ... untranslatable ... errors.

Bugs closed: 192

Reviewed at https://rbcommons.com/s/twitter/r/464/