Implement comprehensive error message mechanism

[anachrocomputer]

Report source file name and line number. Send error report to 'stderr'. Maybe allow for printf-style string formatting.

[anachrocomputer]

Add a function to print a message for an internal compiler error, e.g. in the 'default' position of a 'switch' that should never reach it.

[anachrocomputer]

Make the compiler return EXIT_SUCCESS, or EXIT_FAILURE if 'Error()' is ever called.