Throw an error if invalid path is used with file selects

[alechenninger]

FirefoxDriver won't type something into a file select box that isn't an absolute path. I'm not sure if other drivers work this way, but we should investigate and throw an appropriate exception in cases of unsupported input, instead of simply doing nothing (which is webdriver's default behavior).

[alechenninger]

Meant to put in webdriver module