Error message when checking out a branch when there are local changes
When there are local changes, the error dialog when switching branching is pretty cryptic:
![image](https://user-images.githubusercontent.com/3254314/47982567-7ecd4000-e0d0-11e8-9384-683214daceda.png)
We should show a nicer message with potential steps to fix it: either commit the changes or discard them.