Discovered while testing MC-276813.

Relates to: MC-236295 MC-263121 MC-276750 MC-276751 MC-276813 MC-276828

The bug

If a resource pack defines a texture for a unicode character, and that texture cannot be loaded, the hollow rectangle placeholder texture will be used instead. However, unlike for cases where a texture isn't defined, there will be no error in the log.

How to reproduce

A resource pack which gives the lowercase "a" a 512x9 texture is attached.

1. Download and apply the attached resource pack
2. Watch the log for errors
3. Optionally, compare with a character not defined in the font: 

Expected results

There would be an error indicating that the game couldn't find a valid glyph for this character.

Actual results

No error is produced at all.