qemuBlockStorageSourceGetBackendProps: Report errors on all switch cases

qemuBlockStorageSourceGetBackendProps: Report errors on all switch cases

Few switch cases returned failure but didn't report an error. For a
situation when the backingStore type='volume' was not translated the
following error would occur:

 $ virsh start VM
 error: Failed to start domain VM
 error: An error occurred, but the cause is unknown

After this patch:

 $ virsh start VM
 error: Failed to start domain VM
 error: internal error: storage source pool 'tmp' volume 'pull3.qcow2' is not translated

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>