Extend fontgen.AddGlyphs method return with error types in case of fail

[mbogomolov-ms]

For example for cases:

• The glyph is a whitespace of a kind which we don't recognise
• Glyph index not found in font for codepoint
  And also will be cool, if that problematic symbol will be returned too

We would like to send such errors with context to our bug report systems to quickly locate and fix potential problems