capture loads more errors

Errors which happen while creating a question have their messages prepended with the question number.
Catch 'no right type definition' errors which are really 'variable not defined' errors caused by the evaluator returning undefined variable names untouched.
Catch errors in variable definitions.
Catch errors when evaluating variables.
Catch "variable not defined" and "circular reference" errors when evaluating variables properly, instead of just getting "error in variable definition".
Catch errors which happen before all the queued scripts have been loaded (i.e. which happen in tryInit) - use an alert box to show the error.
In the default theme, the Numbas.die screen hides the stack trace until you click on the error message.