Report encoding detection errors as IOError to prevent fatal errors in P...

Sometimes Java .properties files contain characters in encoding other than US-ASCII or UTF-8. Of course it should be fixed by using native2ascii and escaping such characters.
While loading project to Pootle with many property files it's impossible to find, which file is incorrect, because of badly encoded characters - Pootle show only error about NoneObject at properties.py, but no actual properties filename.
This fix checks if file can be read and if not - raise IOError with filename, so Pootle can read next file, and user can find problematic files in web GUI.