mountfsd: return a better error when invalid file flags are specified on image fd

We already defined an error for this that we return for invalid dir fds
passed in, let's add the same niceness to the image fds.

src/mountfsd/mountwork.c

@@ -353,6 +353,8 @@ static int vl_method_mount_image(
         }
 
         r = validate_image_fd(image_fd, &p);
+        if (r == -EREMOTEIO)
+                return sd_varlink_errorbo(link, "io.systemd.MountFileSystem.BadFileDescriptorFlags", SD_JSON_BUILD_PAIR_STRING("parameter", "imageFileDescriptor"));
         if (r < 0)
                 return r;