Fix: tell the user if a font fails to load and fallback is about to be used

[TrueBrain]

Fixes #7615 (well, not really fixes, but informs the user better).

Motivation / Problem

When fonts fail to load, the user is not informed why and that it failed. Instead, system fallback is loaded, and that is all she wrote. Really frustrating for the user, as he would think he did the path wrong or something. None of that .. if only a single glyph is missing, all the fonts are rejected and the system fallback is used.

Description

So instead of doing stuff in silence, be a bit more talkative:

• Report in console what character failed to render
• Show a GUI error that the font failed, and that the system fallback is used

Hopefully that makes it a bit easier to debug font issues :)

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[LC-Zorg]

(From the perspective of the average gamer)
I have not encountered this problem because...

1. For a long time I didn't know that it is possible to change the font in the game.
2. And when I found out about it, I didn't know how to do it.
3. Later I knew, but today I forgot again how and where.

People familiar with how to change the font will likely benefit from this change. However, the vast majority of players don't know about this possibility.

This proposal is a completely different level of complexity of the work and it is probably not very revealing, but...
The option to select the type and size of fonts available in the settings window would be useful in the game. The current variant x1 / x2 / x4 rarely gives satisfactory results.

[TrueBrain]

(From the perspective of the average gamer)
I have not encountered this problem because...

1. For a long time I didn't know that it is possible to change the font in the game.

2. And when I found out about it, I didn't know how to do it.

3. Later I knew, but today I forgot again how and where.

People familiar with how to change the font will likely benefit from this change. However, the vast majority of players don't know about this possibility.

This proposal is a completely different level of complexity of the work and it is probably not very revealing, but...
The option to select the type and size of fonts available in the settings window would be useful in the game. The current variant x1 / x2 / x4 rarely gives satisfactory results.

Please keep comments relevant to the topic at hand. This Pull Request is, as you already conclude, to fix a specific problem. It is not about fixing [everything else]. If you feel what you describe is a bug, create an issue please, so it can be processed properly. Tnx!

[glx22]

If it can give better debug output than currently (as seen in #7615) let's go