Provide suitable error message for build indexes that are out of bounds (#121)

PR is attempting to patch #121
build_number is usually a str or none, The type would have to be cast to an int when it is defined after being parsed to the scraper.

For Example:
```python
class DailyScraper(Scraper):
    """Class to download a daily build from the Mozilla server"""

    def __init__(self, branch='mozilla-central', build_id=None, date=None,
                 build_number=None, *args, **kwargs):

        self.branch = branch
        self.build_id = int(build_id) //Here
```
I looked at setting the type in optparse however unlike argparse it does not check or convert the input in any way.