FIX: 'resource request errors' close the server.

The test "should handle resource request errors"
was not closing the server at the end of the test,
interfering with the successive tests.
FIX: Check file exist when attempting upload.

Both "page.uploadFile()" and "onFilePicker = function() {}" check file exists before accepting them.
If file don't exists, they are "ignored".

Fixes issue #941: https://code.google.com/p/phantomjs/issues/detail?id=941