Throw a smarter error when a repo can't be found.

When building from a Github repo, if the repo doesn't exist this is the error message:

``` console
Bitovis-MacBook-Pro-2:can-stache-moment-docs bitovi$ documentjs
downloading git://github.com/org/project#master

/Users/bitovi/npm/lib/node_modules/documentjs/node_modules/github-download/node_modules/adm-zip/zipFile.js:66
throw Utils.Errors.INVALID_FORMAT;
^
Invalid or unsupported zip format. No END header found
```

The script fetching the repo should check its validity to give a more-relevant error message.

Run the documented documentjs command against the missing git://github.com/org/project#master repository and inspect the script that fetches the repo, along with the resulting adm-zip error. The work is done when an unavailable repository produces a relevant missing-repository error instead of “Invalid or unsupported zip format.”

Written by the indexing model from the issue text.