Improve ImageFont error messages

[yngvem]

I spent a longer than I'd like to admit trying to figure out why my fonts wouldn't load before I realised that I was using the wrong function. This PR should hopefully help someone else realise their mistake faster.

Changes proposed in this pull request:

• Add more information in the font loading exception messages
• Add links between ImageFont.load and ImageFont.truetype in the documentation

Documentation update:

and similar for truetype:

[radarhere]

Was this also a source of confusion for you? I'm not sure why this wouldn't be suitably obvious.

[yngvem]

No this happened more on auto-pilot while coding in the evening -- I pretty much always try to include the incorrect value in error messages if it fails on input-validation as I find it makes debugging quite a lot easier for me (especially if the error is several frames down the stack from where I'm working).

Feel free to remove it, no hard feelings there :)

[radarhere]

I've created yngvem#1 with some suggestions.

[yngvem]

I just merged the changes now

[yngvem]

I'm unsure why the pipeline crashed, but it seems to be unrelated to the changes in this PR? Maybe just unlucky with the load on the node that was assigned to this pipeline run?

Tests/test_file_tiff.py::TestFileTiff::test_timeout FAILED               [ 48%]

        if not self.map:
            # sort tiles in file order
>           self.tile.sort(key=_tilesort)
E           Failed: Timeout >6.0s

/vpy3/lib/python3.12/site-packages/PIL/ImageFile.py:245: Failed

https://github.com/python-pillow/Pillow/actions/runs/10656884196/job/29544000694?pr=8338

[radarhere]

I've created yngvem#2 with some suggestions.

[yngvem]

Good ideas for changes!

[hugovk]

Thank you!