storage: When fs_mount_volume returns < 0, it's a status_t.

The "mount" command which calls fs_mount_volume direclty handled this
properly, but this class did not; which meant that user-visible error
messages about partitions failing to mount just said "general system error"
instead of the real one.

Fixes #14540.

src/kits/storage/disk_device/Partition.cpp

@@ -572,7 +572,7 @@ BPartition::Mount(const char* mountPoint, uint32 mountFlags,
 	if (device >= 0)
 		return Device()->Update();
 
-	return B_ERROR;
+	return device;
 }