Raise NotFoundError if downloading from a URL without specifying a path (#89)

Example output:

```bash
$ mozdownload --url=http://mozqa.com
Traceback (most recent call last):
  File "/Users/dhunt/.virtualenvs/mozdownload/bin/mozdownload", line 8, in <module>
    load_entry_point('mozdownload==1.7.2', 'console_scripts', 'mozdownload')()
  File "/Users/dhunt/workspace/mozdownload/mozdownload/scraper.py", line 939, in cli
    build.download()
  File "/Users/dhunt/workspace/mozdownload/mozdownload/scraper.py", line 233, in download
    if os.path.isfile(os.path.abspath(self.target)):
  File "/Users/dhunt/workspace/mozdownload/mozdownload/scraper.py", line 452, in target
    raise NotFoundError('There is no file to be downloaded', self.final_url)
mozdownload.scraper.NotFoundError: There is no file to be downloaded: http://mozqa.com
```