Better "duplicate location" error

[simoncozens]

Before:

$ fontc test.glyphs
[2025-11-11T15:10:13.213365Z ThreadId(1) fontc ERROR] Invalid source glyph 'ga-ethiopic':
'duplicate location Normalized {wdth: 0.60, wght: 0.40}'

After:

$ fontc test.glyphs
[2025-11-11T15:12:28.375830Z ThreadId(1) fontc ERROR] Invalid source glyph 'ga-ethiopic':
'layer eb4070ae-ad02-46bb-9ea2-ac5b398e3b7c provided outline at wdth=100, wght=400
but an outline for this location was already defined'

[cmyr]

hm, I do think you hit on something important here, which is that it would be nice to delegate more of the error details to the crates that know things about sources. I think ultimately I'd prefer to have the backend define its own error enum or something, but that's a more invasive change, so i'm happy to merge this if it concretely improves diagnostics.