Qmlplugindump error messages are not written consistently

For errors in command line parameters qmplugindump uses cerr
but errors during plugin loading are logged using qWarning().
It would be better to log always to cerr in case of problem
because some platforms (BB10) have logging from Qt redirected.

Task-number: QTBUG-36378
Change-Id: I04506c2f12b87190966e66c482f4ac5aed44af05
Signed-off-by: Daniel Pesch <dpesch@blackberry.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
Fix warning about comparing double to boolean in  js_equal().

jsapi\qjsvalue.cpp(749) : warning C4805: '==' : unsafe mix of type 'double' and type 'bool' in operation

Change-Id: I06d0950ce932ea635c422d3fb42ca80e6725dda1
Reviewed-by: Lars Knoll <lars.knoll@digia.com>