Distinguish ICEs from normal compiler errors

[Aaron1011]

It's often useful to distinguish compiler ICEs, which are bug by definition, from compiler errors, which are usually not bugs. This is especially true when performing rustdoc crater runs, as any compiler errors encountered will be unrelated to rustdoc itself.

[Skgland]

This is to my knowledge already implemented[^code], but currently at least partially broken[^bad].

[^code]: Ice in root crate https://github.com/rust-lang/crater/blob/master/src/runner/test.rs#L185-L187, Ice in dep https://github.com/rust-lang/crater/blob/master/src/runner/test.rs#L195-L199 [^bad]: 19 out of 20 root regressions in pr-135272 appear to be ICEs, all of which got categorized as build failed (unknown)