QmlDesigner.Model: allowing some diagnostics for exceptions

Setting QTCREATOR_QTQUICKDESIGNER_WARN_EXCEPTION will give detailed
information about any exception thrown.

Change-Id: Ia11b98324e6f0c27e086778ddcb1afec67d92e6a
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>