Option to compile as much as possible / show all errors

Hi there!

When I'm using fontc, I'm aware it's an incomplete and imperfect font compiler, and I'm willing to make sacrifices in my source (or hacks on fontc's source itself) to try and get all the way to a TTF.

One problem I'm running into with this is that fontc only reports a few errors at a time (as far as I can tell), instead of all* errors at once. This means that I can't solve classes of errors in one go, I have to do a fix for 3-4 glyphs, then run fontc again, get the same error for a different 3-4 glyphs, fix those, rinse & repeat ad nauseam.

I understand in the long run failing fast is probably best and a sensible default, however much like Cargo's proposed [`--keep-going`](https://github.com/rust-lang/cargo/issues/10496), I think something similar could help fontc's early adopters work through the teething pains and get a TTF built.

(*Obviously there may be some point at which no further work can be done due to the accumulated errors, but that's still distinct from stopping everything when the first error is discovered)