qt: Prevent non-functional GUI from popping up during Init

When a InitError or InitWarning happens, the
GUI pops up but is unusable (until Init finishes).

This is caused by showNormalIfMinimized. Add a message
flag to skip this call for Init errors or warnings.
Merge pull request #3427

1ad2636 qt: Prevent non-functional GUI from popping up during Init (Wladimir J. van der Laan)