Warning on Sync page if other users have changes

[isc-pbarton]

Probably a low priority because basic mode is intended for single-developer namespaces, so ideally other users will not have any active changes.

If other users have changes in the namespace, the sync will fail to merge the remote trunk branch because there are unstaged changes in the repository. It's a little hard to tell what's going on from the output. We should add a warning message to the sync page that says that another user has unstaged changes, so merging the remote branch will fail.