Stop using most error flags in mercury_compile_front_end.m.

compiler/mercury_compile_front_end.m:
    At the top levels of this predicate, stop using boolean flags
    that are not directly derived from the presence/absence of errors
    in a list of error_specs.

    Likewise, stop paying attention to the exit status; instead,
    pay attention to the things that would *cause* a nonzero exit status.

compiler/post_typecheck.m:
    Instead of counting the NUMBER of unproven type class constraints,
    record the error_specs we generate for such constraints.
    Since a single error_spec can report any strictly positive number
    of unproven constraints in a context, their number will be lower,
    but the only test we care about is "are there unproven constraints?",
    and for this, the error_spec list works.

compiler/typecheck.m:
    Record the error_spec that reports exceeding the iteration limit,
    to simplify the correctness argument of the code that deals with
    that result.