Allow by-string matching in throwError

As of today the `throwError` matcher accepts arguments of type being either:
- a regexp
- a function

Personally I find myself quite often matching the error message literally and in this case escaping regexp special chars is a bit of chore. Could we also accept strings as arguments and use the == / === operator to match this string against the error message?

This would definitivelly make my life a bit easier, happy to send a pull request if this meakes sense.

Start at the throwError matcher entry point and review how regexp and function arguments are currently handled. Done means string arguments match the error message literally using the requested equality semantics, with coverage added to the relevant matcher tests.

Written by the indexing model from the issue text.