Avoid a divide-by-zero on malformed BTG files.

Avoid a divide-by-zero on malformed BTG files.

Encountered while testing v850 airports; some airports
generate materials with no tris, and hence no indices.
This causes a divide-by-zero when computing the index stride.

Detect this, and convert the BTG reader to throw exceptions
in error conditions, and to catch this and report the
appropriate result code.