Data transfer was not checking if the required geometry existed, thus
causing a segfault when it didn't. This adds the required checks, and
reports errors if geometry is missing.
This also replaces instances of the words "polygon" and "loop" in error
messages with "face" and "corner" respectively, to be consistent with
the rest of the existing UI.
Reviewed By: mont29
Differential Revision: http://developer.blender.org/D2410
2e15618f49