Fail more descriptively if you try to switch branches with conflicting uncommitted work

[isc-tleavitt]

Right now, in basic mode, if I try to switch branches and it fails with something like:

error: Your local changes to the following files would be overwritten by checkout:
src/HCC/Foo/LearningServicesDemo.cls
Please commit your changes or stash them before you switch branches.
Aborting

This just gets buried in source control message output. We should fail more helpfully (e.g., with an alert) instead and make it clear to the user that the branch hasn't actually changed.