Improve error messages for `/cfunction`

[Earthcomputer]

Currently, if the user makes a mistake in their function file, such as by adding a slash before the function name, like so:

/csnap ~0.5 ~ ~

Then clientcommands reports "unknown or incomplete command" on the /cfunction command in chat itself:

This is very misleading for users and a better error message should be created.

[morgantic]

I just ran into this issue and give a very big +1 on this, as someone who's not very skilled in minecraft command language, this would be extremely helpful!