compiletest needs to check error code for compile-fail tests

[brson]

Like run-fail tests, we need to make sure the compiler not only fails, but fails correctly. Currently the following tests all fail by segfaulting:

    [compile-fail] ../src/test/compile-fail/break-outside-loop.rs
    [compile-fail] ../src/test/compile-fail/dup-link-name.rs
    [compile-fail] ../src/test/compile-fail/ext-nonexistent.rs
    [compile-fail] ../src/test/compile-fail/extenv-no-args.rs
    [compile-fail] ../src/test/compile-fail/extenv-not-string-literal.rs
    [compile-fail] ../src/test/compile-fail/extenv-too-many-args.rs
    [compile-fail] ../src/test/compile-fail/extfmt-missing-type.rs
    [compile-fail] ../src/test/compile-fail/extfmt-no-args.rs
    [compile-fail] ../src/test/compile-fail/extfmt-non-literal.rs
    [compile-fail] ../src/test/compile-fail/extfmt-non-literal2.rs
    [compile-fail] ../src/test/compile-fail/extfmt-not-enough-args.rs
    [compile-fail] ../src/test/compile-fail/extfmt-too-many-args.rs
    [compile-fail] ../src/test/compile-fail/extfmt-unknown-type.rs
    [compile-fail] ../src/test/compile-fail/extfmt-unsigned-plus.rs
    [compile-fail] ../src/test/compile-fail/extfmt-unsigned-space.rs
    [compile-fail] ../src/test/compile-fail/extfmt-unterminated-conv.rs
    [compile-fail] ../src/test/compile-fail/macro-2.rs
    [compile-fail] ../src/test/compile-fail/macro.rs

[brson]

Had to revert c42401e