Deprecated use of raising error with tuple

[Jay Pipes]

r79 introduced a warning from pep8:

glance/server.py:321:27: W602 deprecated form of raising exception
            raise exc_type, exc_value, exc_traceback
                          ^
    When raising an exception, use "raise ValueError('message')"
    instead of the older form "raise ValueError, 'message'".

    The paren-using form is preferred because when the exception arguments
    are long or include string formatting, you don't need to use line
    continuation characters thanks to the containing parentheses. The older
    form will be removed in Python 3000.