font/sfnt: add ErrColoredGlyph.

Also add tests for the Noto proprietary fonts. Prior to this commit,
NotoColorEmoji.ttf was unsupported. It's still not well supported, but
the error message returned is now more informative.

Change-Id: I61a3301d7f2458a4b838eb1de7a73d6472e3486f
Reviewed-on: https://go-review.googlesource.com/43694
Reviewed-by: David Crawshaw <crawshaw@golang.org>
2 files changed