Better require error messages in comptime

[edemaine]

By the way, I'm thinking throws should allow for specifying and checking what error message we get. What do you think would be the ideal format (presumably something after ---)? Perhaps the exact contents of error.toString(), which seems to be of the form type: message. Alternatively, we might need regexes in some contexts, if the error message varies, but I can't think of a setting where that's the case.

[STRd6]

Adding an optional exact error message to match to throws sounds good, something like:

throws ```
test name
---
...
---
exact error message

And allow omitting the final `---` to accept any error message.