Tools > File Editor should give more descriptive error messages

[moorscode]

What did you expect to happen?

Have the file editor tell me I don't have the right privileges / rights to edit the file.

What happened instead?

Tells me I 'cannot create' the file.

How can we reproduce this behavior?

Log in on a user without edit_files permissions and go to the Tools > File Editor page.

Suggested solution

If the current user doesn't have the rights, make it clear that that's the problem.
If the file cannot be written because of a Filesystem problem, notify the user of that as well (this is currently not implemented at all).