Fix error message for undefined Java JDK path.
Fix error message for undefined Java JDK path.

Change-Id: I05a4fea7f043d45c40778303dc914659cd37a65c
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@theqtcompany.com>

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