More helpful errors

Some error messages could be more helpful in that they could give more of a hint how to resolve them. E.g. @cdwensley just run into
```
../ReleaseTools/release --token ...(omitted)...
ERROR: could not find 'webdir' with clone of your gh-pages branch
```
This could give a hint how to resolve it (even if it just points at a part of the README.md).