Remove QV4::DiagnosticMessage

Remove QV4::DiagnosticMessage

QQmlError provides the same functionality, so let's rather
use that where required. Remove the dependency of
codegen onto the ExecutionContext that was only
required for error handling.

Change-Id: Ib0b61c0e138f89ff989c32996c93c339e4b62223
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>