feat(font)!: correct error propagation in font generation

[rami3l]

In response to #93, this output format is long overdue:

> cargo run
   Compiling monoxide v0.1.0 (/work/monoxide)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.30s
     Running `target/debug/monoxide`
Hello from Monoxide!
Error: Failed to layout glyphs

Caused by:
    0: Failed to evaluate glyph, at glyph index 6
    1: bezier stroking unimplemented at (NoId): try putting `.transformed()` before `.stroked()`