V8DebuggerClient: Enable breakpoints on Exceptions

V8DebuggerClient: Enable breakpoints on Exceptions

Previously break on exceptions was always enabled. Now, the user
can add a breakpoint on Javascript Exceptions from the breakpoint
window in the debugger.

Change-Id: Ia9ee813d2fccf89b607fe1dcbad47b4084155a48
Reviewed-on: http://codereview.qt-project.org/6348
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>