Rate limit not notified

[seblaz]

When the rate limit is reached, the user isn't notified. Instead the following error ocurrs:
TypeError: Cannot read property 'toLowerCase' of undefined
at (...)/node_modules/github-metadata/index.js:202:43
at process.internalTickCallback (internal/process/next_tick.js:77:7)

This happens because the data retrieved doesn't contain the repository.name property. I think it'll be better to let the user know why this error is happening.

[doowb]

Thanks for the issue.

If you'd like to submit a PR throw an error when the rate limit is reached, I'll review it. Otherwise, I'll try to get to it when I have a chance.