Add -D fail-fast

[kLabz]

Often when dealing with a wall of errors, only the first one really makes sense.

Compiling with -D fail-fast, compilation will abort on first error (including its "sub errors") to skip the noise.

[Simn]

I really like this, I'd even make it the default and see if somebody complains, but for now this is good enough.

Should the define be added to the list of defines that doesn't change context signature?

[kLabz]

Should the define be added to the list of defines that doesn't change context signature?

Good point!