[UI] Fix usage of 'with Image.open' in trackericons

* Revert changes made to fix 'too many files open' as Image.open does
   not return a file descriptor and generated the following error:
     exceptions.AttributeError: 'NoneType' object has no attribute 'startswith'
 * Also fix style for raising an exception.

deluge/ui/tracker_icons.py

@@ -359,13 +359,12 @@ class TrackerIcons(Component):
 
         if PIL_INSTALLED:
             try:
-                with Image.open(icon_name):
-                    pass
+                Image.open(icon_name)
             except IOError, e:
                 raise InvalidIconError(e)
         else:
             if os.stat(icon_name).st_size == 0L:
-                raise InvalidIconError, "empty icon"
+                raise InvalidIconError("empty icon")
 
         return icon_name
 
@@ -434,7 +433,7 @@ class TrackerIcons(Component):
         """
         if icon:
             filename = icon.get_filename()
-            with Image.open(filename) as img:
+            img = Image.open(filename)
             if img.size > (16, 16):
                 new_filename = filename.rpartition('.')[0]+".png"
                 img = img.resize((16, 16), Image.ANTIALIAS)