Handle common errors

- [x] Could not find `.all-contributorsrc` file.
- [x] `projectOwner` is not set in `.all-contributorsrc` file.
- [x] `projectName` is not set in `.all-contributorsrc` file.
- [ ] `files` was overriden in `.all-contributorsrc` file and is empty.
- [ ] Could not find some of the files to inject into. If adding a contributor, the addition should still take place.
- [ ] Could not find any injection tags. If adding a contributor, the addition should still take place.
- [x] No login when adding a contributor.
- [ ] No contribution types when adding a contributor.
- [x] Login not found when adding a contributor.
- [ ] Network error adding a contributor.
- [ ] Contribution type not found.

In all of these cases: Should print some helpful error and exit.

If you wish to contribute, first off: thanks!
Feel free to tackle one or a few of these problems, no need to do them all (considering the number of them, I'd actually prefer multiple PRs).