fix screenshot path env var handling

There is a small bug in the way that the SCREENSHOT_PATH variable is handled in FailureHelpers#screenshot.

It forces users to define the variable like this:

```
SCREENSHOT_PATH=screenshots/
```

or end up with screenshots in their project with names like this:

```
screenhotsscreenshot_0.png
```

Another problem is that if the directory indicated by SCREENSHOT_PATH does not exist, we throw this error:

```
Errno::ENOENT: No such file or directory @ rb_sysopen - /path/that/does/not/exist
```

I fixed the path problem and raise an error with a more helpful message.