Make failure to store the build graph a proper error
Make failure to store the build graph a proper error

If we cannot store the build graph, then that's a hard error. It does
not make sense to just emit a warning.

Change-Id: Ia4a586c80354266e61a0842b00197eae466c9749
Reviewed-by: Christian Stenger <accbb51712d7b9c4fb108439d01e716148e5f9e6@qt.io>

qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs