Cleanup exception handling

Cleanup exception handling

The code in the Exception class operates entirely on the engine's data,
so move it into ExecutionEngine instead. This eliminates the need for
a QV4::Exception class and catches and old code that tries to still do
catch (Exception &) instead of catch (...)

Change-Id: Ie608bec6af652038aca6c9423c225a4d7eb13b39
Reviewed-by: Lars Knoll <lars.knoll@digia.com>