Improve image loading
Improve image loading

* Except more errors that PIL.Image can product. When it is loading a
  bad png file, it can fail not only with an IOError but with
  a struct.error, OverflowError and, strangely enough, SyntaxError.
* If the image fails to load, try to load it using Pixbuf. There are some
  cases that PIL will fail to open but Pixbuf will be successful.

nowsecure/datagrid-gtk3,jcollado/datagrid-gtk3