Improved mechanism for compilation exit code

The current mechanism for determining the exit code at the end of compilation is based on the final compilation output, which is a pair of strings corresponding to the `stdout` and `stderr` output. If the final `stderr` output starts with `###`, which is printed at the beginning of error messages, the compiler will have a non-zero exit code. (See [condition](https://github.com/CakeML/cakeml/blob/e9cafe6795f84d7458975af3f196d67c126c9763/compiler/compilerScript.sml#L321) and its [application](https://github.com/CakeML/cakeml/blob/e9cafe6795f84d7458975af3f196d67c126c9763/compiler/bootstrap/translation/compiler32ProgScript.sml#L334)). Most failures in the compilation stages cause such an error message. Separately to this, all current success cases provide non-empty `stdout` output and all current failure cases provide empty `stdout` output.
In general, it would be nice if this mechanism was not so dependent on this specific prefix, and the final compilation output was more explicit about which exit code applied.