n64tool: return error in case the seek offset required backward seek.
n64tool: return error in case the seek offset required backward seek.

For instance, if the user asks to put a DFS at 1M, but the code/data is
2M, it simply can't be done. Instead of silently putting the DFS at 2M,
better return an error and let the user handle it, especially since
the seek offset needs to be passed to dfs_init() so the user needs
to be aware of the correct value.

Fixes #141

DragonMinded/libdragon,sp1187/libdragon,DragonMinded/libdragon,sp1187/libdragon,DragonMinded/libdragon,sp1187/libdragon