This commit re-works how the monitor() function works and how it both receives
and transmits errors. There are a few cases in which the compiler can abort:
1. A normal compiler error. In this case, the compiler raises a FatalError as
the failure value of the task. If this happens, then the monitor task does
nothing. It ignores all stderr output of the child task and it also
suppresses the failure message of the main task itself. This means that on a
normal compiler error just the error message itself is printed.
2. A normal internal compiler error. These are invoked from sess.span_bug() and
friends. In these cases, they follow the same path (raising a FatalError),
but they will also print an ICE message which has a URL to go report a bug.
3. An actual compiler bug. This happens whenever anything calls fail!() instead
of going through the session itself. In this case, we print out stuff about
RUST_LOG=2 and we by default capture all stderr and print via warn!() so it's
only printed out with the RUST_LOG var set.
d0f6ef080b