Better error handling for uploads and renaming

Hello all.

I installed WebFM on my Drupal 6.4 site and it works great. There were only 2 things I needed to change. I have included a patch for your review.

1. When the user clicks the Upload button without selecting a file, the webfm module by default will give a rather cryptic error that normal users wouldn't be able to interpret. Something about a function call that bombed out... etc. I needed to change this to simply prompt the user to click "Browse" and select a file and try again. So I added some new logic to the webfm_upload function where other errors are caught.

2. The rename function wasn't checking to see if the user was using a '/' slash in the file name and returning an accurate error about illegal characters. The default behavior wasn't causing any damage. It just wasn't giving an accurate error message about whether a '/' slash is okay in a filename. So I changed webfm_ajax() function under the rename switch case to accomodate this.

A patch file is attached. Please integrate my changes (or something that accomplishes the same thing) into your next release.

-- jcwatson11