font-patcher: Check if glyph source is available (`--glyphdownload`)

[Finii]

[why] When users just download the script (and not the source glyphs) the script fails with an obscure error message.

[how] Check if the glyphdir exists at all. If not give a hint to download the glyphs.

Check if the individual glyph font exists and is readable. Bail out if not.

Add option --glyphdownload that tries to download missing glyph fonts.

Requirements / Checklist

• [x] Read the Contributing Guidelines
• [x] Verified the license of any newly added font, glyph, or glyph set

What does this Pull Request (PR) do?

Add checks before we open a symbol font and give hints to the users if we can not.

How should this be manually tested?

• Patch a font (shall work as before)
• Remove one (needed) symbol font, patch a font again (errors out)
• Remove complete src/glyphs/ (or use --glyphdir), patch a font again (errors out)

The two bottom cases a meaningful message should be printed.

• Repeat the 3 tests but specify --glyphdownload. (all shall work)

Any background context you can provide?

For example #659 #662 #495

What are the relevant tickets (if any)?

Screenshots (if appropriate or helpful)

Edit: Add --glyphdownload as that has been added as 2nd commit to the PR

[Finii]

Not sure if we want to go that route, but it is requested a lot it seems...:

Autodownload...

[Finii]

Two notes:

• Of course the first commit should be merged regardless
• The second commit is kind of optional

If we go the route with the second commit there might be problems of font-patcher version versus actual repo version - when the glyph paths change. So in reality we should download the repo-glyph stuff from the version (and not MASTER) that the font-patcher actually is. But this is not possible at the moment because the versions are not increased even when the directory structure is changed. This might need more thinking and/or ideas. At least maybe a warning in the code when a download fails, that one needs to use the font-patcher HEAD.

[Finii]

Rebase on master, force push.

One of the original commits is already in master.

[Finii]

All readme files need to be corrected :stop_sign:

[Finii]

Does this download the --makegroups stuff also? I guess not. Would that be beneficial, or just disable the feature? Who uses that feature?

[Finii]

I believe we should promote the FontPatcher.zip instead. This would need to keep a parallel set of sub-files as for the zip generation. A lot of maintenance burden.