Tests that pause on exception works.

Script source was shown.
Script execution paused.
Call stack:
    0) throwAnException (debugger-pause-on-exception.js:16)
    1) handleClick (debugger-pause-on-exception.js:21)
    [setTimeout]
    0)  (debugger-pause-on-exception.js:35)
Paused on exceptionReferenceError: unknown_var is not defined
Script execution resumed.