enhance error message for the JS function File.copy
enhance error message for the JS function File.copy

Now we even see *why* a copy operation failed.

Change-Id: I1d4feb91d1387e883c35d841391236fad97c13e8
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@digia.com>

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