don't use bare `except:` (per PEP-8)

Per [PEP8](http://www.python.org/dev/peps/pep-0008/):

> A bare except: clause will catch SystemExit and KeyboardInterrupt exceptions, making it harder to interrupt a program with Control-C, and can disguise other problems. If you want to catch all exceptions that signal program errors, use except Exception: (bare except is equivalent to except BaseException:).