Initialise stack trace of com.qwirx.test.FakeBrowserEvent.UnexpectedExceptionThrown.

Set it to the stack trace of the unexpected exception. This is likely to
be the most useful place to start debugging.
Add more correctness assertions.

Improve debugging messages.
Improve control over assertEvents responses.

Allow returning true or false, and stopping propagation of events in
handlers.

Rename opt_continue_if_exception_not_thrown because this is an event,
not an exception.