Blender Git Loki

Fix T50216: Missing checks caused data transfer segfault

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: https://developer.blender.org/D2410