Skip autocorrect invalid source files

[keith]

We recently hit this issue #2823 which incorrectly formats files when they have some invalid syntax. While we could surely fix this specific case, through some discussion about the ideal solution here, we thought it might make sense to skip autocorrection (and warn / error) when a file is invalid.

We believe that sourcekit returns this info, so that might be enough to provide to the user.

[jpsim]

Done in #3349