Return an error if we try to write a file to /

[rubiojr]

Fixes #43

[rubiojr]

Should the client also append the path when no path is specified? i.e.

charm fs cp hello.txt charm:/

should create /hello.txt perhaps?

[rubiojr]

The client does the file path encryption before sending it to the server, so perhaps I could use more realistic paths in the tests I added, using EncryptPath from the fs package? Given EncryptPath also splits the path on / encrypting path components individually, don't think it makes a big difference it this particular case.

[toby]

This is awesome! Great catch and thank you so much for the contributions @rubiojr!!