Adjust compilation-mode regexps.

- Require that errors start in first column (after optional leading ;).

- Don't match on profiler messages.

As a result, we don't match on the indented "context...:" items in a
noisy Typed Racket error message. (Those error messages get printed
directly -- they don't go through error-display-handler -- so we can't
limit the context in the usual way. This commit at least avoids adding
insult to injury, by not linkifying all those context srclocs.)