distgit: Don't raise, return the error to the client

Raising only sends the error to the logs, the client thinks everything
went fine.

roles/distgit/files/dist-git-upload.cgi

@@ -186,7 +186,7 @@ def main():
 
         except OSError as e:
             if e.errno != errno.EEXIST:
-                raise e
+                send_error(str(e))
 
         try:
             os.link(dest_file, old_path)