Debugger: Output warnings when preferred debugger is not set up.
Debugger: Output warnings when preferred debugger is not set up.

Reviewed-by: hjk
Task-number: QTCREATORBUG-3990

maui-packages/qt-creator,farseerri/git_code,maui-packages/qt-creator,xianian/qt-creator,danimo/qt-creator,AltarBeastiful/qt-creator,ostash/qt-creator-i18n-uk,renatofilho/QtCreator,sandsmark/qtcreator-minimap,KDE/android-qt-creator,darksylinc/qt-creator,martyone/sailfish-qtcreator,amyvmiwei/qt-creator,malikcjm/qtcreator,pcacjr/qt-creator,AltarBeastiful/qt-creator,pcacjr/qt-creator,martyone/sailfish-qtcreator,syntheticpp/qt-creator,amyvmiwei/qt-creator,Distrotech/qtcreator,ostash/qt-creator-i18n-uk,jonnor/qt-creator,kuba1/qtcreator,martyone/sailfish-qtcreator,KDE/android-qt-creator,malikcjm/qtcreator,sandsmark/qtcreator-minimap,colede/qtcreator,dmik/qt-creator-os2,malikcjm/qtcreator,amyvmiwei/qt-creator,dmik/qt-creator-os2,danimo/qt-creator,pcacjr/qt-creator,jonnor/qt-creator,Distrotech/qtcreator,azat/qtcreator,KDE/android-qt-creator,xianian/qt-creator,kuba1/qtcreator,martyone/sailfish-qtcreator,jonnor/qt-creator,bakaiadam/collaborative_qt_creator,pcacjr/qt-creator,xianian/qt-creator,martyone/sailfish-qtcreator,KDE/android-qt-creator,bakaiadam/collaborative_qt_creator,kuba1/qtcreator,pcacjr/qt-creator,amyvmiwei/qt-creator,sandsmark/qtcreator-minimap,malikcjm/qtcreator,Distrotech/qtcreator,bakaiadam/collaborative_qt_creator,malikcjm/qtcreator,KDE/android-qt-creator,omniacreator/qtcreator,xianian/qt-creator,dmik/qt-creator-os2,colede/qtcreator,richardmg/qtcreator,KDAB/KDAB-Creator,danimo/qt-creator,dmik/qt-creator-os2,renatofilho/QtCreator,hdweiss/qt-creator-visualizer,farseerri/git_code,farseerri/git_code,jonnor/qt-creator,KDAB/KDAB-Creator,jonnor/qt-creator,duythanhphan/qt-creator,amyvmiwei/qt-creator,Distrotech/qtcreator,farseerri/git_code,omniacreator/qtcreator,richardmg/qtcreator,Distrotech/qtcreator,colede/qtcreator,syntheticpp/qt-creator,omniacreator/qtcreator,bakaiadam/collaborative_qt_creator,KDE/android-qt-creator,dmik/qt-creator-os2,syntheticpp/qt-creator,danimo/qt-creator,duythanhphan/qt-creator,hdweiss/qt-creator-visualizer,sandsmark/qtcreator-minimap,colede/qtcreator,xianian/qt-creator,darksylinc/qt-creator,KDAB/KDAB-Creator,renatofilho/QtCreator,colede/qtcreator,renatofilho/QtCreator,xianian/qt-creator,pcacjr/qt-creator,renatofilho/QtCreator,farseerri/git_code,omniacreator/qtcreator,richardmg/qtcreator,colede/qtcreator,amyvmiwei/qt-creator,AltarBeastiful/qt-creator,farseerri/git_code,Distrotech/qtcreator,darksylinc/qt-creator,renatofilho/QtCreator,sandsmark/qtcreator-minimap,xianian/qt-creator,danimo/qt-creator,maui-packages/qt-creator,AltarBeastiful/qt-creator,AltarBeastiful/qt-creator,hdweiss/qt-creator-visualizer,maui-packages/qt-creator,maui-packages/qt-creator,KDAB/KDAB-Creator,ostash/qt-creator-i18n-uk,farseerri/git_code,kuba1/qtcreator,maui-packages/qt-creator,darksylinc/qt-creator,omniacreator/qtcreator,malikcjm/qtcreator,KDE/android-qt-creator,farseerri/git_code,xianian/qt-creator,ostash/qt-creator-i18n-uk,richardmg/qtcreator,syntheticpp/qt-creator,richardmg/qtcreator,danimo/qt-creator,danimo/qt-creator,sandsmark/qtcreator-minimap,KDAB/KDAB-Creator,Distrotech/qtcreator,AltarBeastiful/qt-creator,omniacreator/qtcreator,dmik/qt-creator-os2,KDAB/KDAB-Creator,kuba1/qtcreator,amyvmiwei/qt-creator,hdweiss/qt-creator-visualizer,danimo/qt-creator,ostash/qt-creator-i18n-uk,kuba1/qtcreator,hdweiss/qt-creator-visualizer,maui-packages/qt-creator,hdweiss/qt-creator-visualizer,darksylinc/qt-creator,martyone/sailfish-qtcreator,AltarBeastiful/qt-creator,azat/qtcreator,dmik/qt-creator-os2,darksylinc/qt-creator,azat/qtcreator,duythanhphan/qt-creator,syntheticpp/qt-creator,omniacreator/qtcreator,bakaiadam/collaborative_qt_creator,jonnor/qt-creator,martyone/sailfish-qtcreator,azat/qtcreator,ostash/qt-creator-i18n-uk,syntheticpp/qt-creator,bakaiadam/collaborative_qt_creator,duythanhphan/qt-creator,kuba1/qtcreator,darksylinc/qt-creator,richardmg/qtcreator,syntheticpp/qt-creator,danimo/qt-creator,duythanhphan/qt-creator,martyone/sailfish-qtcreator,richardmg/qtcreator,kuba1/qtcreator,darksylinc/qt-creator,martyone/sailfish-qtcreator,malikcjm/qtcreator,azat/qtcreator,duythanhphan/qt-creator,colede/qtcreator,duythanhphan/qt-creator,azat/qtcreator,xianian/qt-creator,kuba1/qtcreator,ostash/qt-creator-i18n-uk,amyvmiwei/qt-creator,KDE/android-qt-creator,bakaiadam/collaborative_qt_creator,pcacjr/qt-creator,AltarBeastiful/qt-creator