gracefully

If the 'Include Custom Normals' import option is used, and the FBX looks
like it has LayerElementNormal (but lacks the actual data), dont throw
an error, just print a warning and continue.

(this has a downside, since such stuff can easily go unnoticed then, but
this seems to be done elsewhere as well...)

Maniphest Tasks: T78083

Differential Revision: https://developer.blender.org/D8122