qemu-img: Improve commit invalid base message

When trying to invoke qemu-img commit with a base image file name that
is not part of the top image's backing chain, the user receives a rather
plain "Base not found" error message. This is not really helpful because
it does not explain what "not found" means, potentially leaving the user
wondering why qemu cannot find a file despite it clearly existing in the
file system.

Improve the error message by clarifying that "not found" means "not
found in the top image's backing chain".

Reported-by: Ala Hino <ahino@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20161201020508.24417-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

qemu-img.c

@@ -912,7 +912,9 @@ static int img_commit(int argc, char **argv)
     if (base) {
         base_bs = bdrv_find_backing_image(bs, base);
         if (!base_bs) {
-            error_setg(&local_err, QERR_BASE_NOT_FOUND, base);
+            error_setg(&local_err,
+                       "Did not find '%s' in the backing chain of '%s'",
+                       base, filename);
             goto done;
         }
     } else {