Improve error messages in mw.html

Add more information to error messages in mw.html. This includes the
error level, the function name, and the position of the argument in the
argument list. Where possible, use the functions in libraryUtil.lua to
do this.

Some functions in mw.html accept multiple types, so add a checkTypeMulti
function to libraryUtil.lua to make these kinds of functions easy to check.
And while we're at it, add test cases for libraryUtil.lua as well.

Change-Id: If9cf9a52bd4b1bb42cc7f9f1f1096828710cbc52