Unify error reporting for configure

Move last two tests (submodule and nowhitespace) to configure
system and unify error reporting. Now all error
messages come from one single place qtwebengine_checkErrors().
This function is used by configure system reporting and by make call.
Remove duplicated strings.

Add extra messages when module is not going to be built.

Change-Id: Ib373facd58135325495aad52b6e600ec9a61f31f
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>