Add missing format parameter in error message

Hello,

When making a mistake on the module:testname syntax, I encountered a stray `'%s'` in an error message:

    $ ./bin/nosetests nose.a:b:c
    [...]
    ValueError: Test name '%s' could not be parsed. Please format test names as path:callable or module:callable.

This turns it into:

    $ ./bin/nosetests nose.a:b:c
    [...]
    ValueError: Test name 'nose.a:b:c' could not be parsed. Please format test names as path:callable or module:callable.

Thanks for your work.