Clean up internal errors

[Matt Giuca]

Internal errors in Mars are currently quite horrible. They shouldn't happen at all, but since this is an early project, they tend to happen a lot. So they should be a lot more graceful than they currently are.

- Some code calls require.error/1, which raises a software_error (this is preferred). This includes all of the Mercury standard library, which we occasionally get errors from as well.
- Some code calls exception.throw/1, usually with a string. This is bad -- you shouldn't throw string exceptions. Replace all of these with calls to require.error/1.
- Exceptions are caught and rethrown several times (depending on where they are thrown from). Exceptions are rethrown with exception.throw/1, which wraps them in a univ_cons, rather than exception.rethrow/1, which preserves their original type. This causes ugly errors to be printed with lots of univ_conses and then a big string literal, whereas a software_error prints nicely. Find all rethrow points and use exception.rethrow/1 instead.
- Exceptions at the top level are caught and displayed specially by us if they are context.errors, but otherwise propagate to the top. The only other exceptions should be software_error, which are handled well by Mercury, but we should handle them better. Catch, reprint, and display the text "This error indicates a bug in Mars. Please report it (https://bugs.launchpad.net/mars/+filebug)."
- Handle software errors in the interactive prompt without crashing the entire program (print the error and continue the interactive session).

[Matt Giuca (mgiuca)]

Rethrow points are:
- mars.m: main
- marsc.m: main
- interactive.m: prompts_st, do_user_command, exec_stmt_internal

[Matt Giuca (mgiuca)]

Fixed in trunk r995, 996, 998, 999 and 1000.