Improved mechanism for compilation exit code

[halogentlepersuasion]

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 and its application). 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.

[ordinarymath]

I should be able to do this in since I'm going to be touching nearby code for #355 (comment)