Fix #11.

File is verified via `fs.existsSync` before the phantom `uploadFile` method is called.
Passes error to `run` callback. Previously, the run callback `err` attribute was hard coded to null.
Fixed a bug in `untilOnPage`, where it was returning when it shouldn't have been.
seems this would be cleaner as:

```
if (result === value) {
...
}
else if (diff > timeout) {
...
}
else {
...
}