Make the check for the `message_extractors` setup keyword more robst.
Make the check for the `message_extractors` setup keyword more robst.

git-svn-id: c0ffdcf8c2e87c8872995a0c731507214cc45958@124 59ecc08e-a131-0410-9ea7-d4c0f28ac310

Phaiax/python-babel,miracle2k/babel,cc-archive/babel,mbr/Babel-CLDR,mbr/Babel-CLDR,miracle2k/babel,joshfinnie/Babel,joshfinnie/Babel,Phaiax/python-babel,cc-archive/babel