pre-sync verification and error messages

[ashleyswang]

It would be nice to add some verification before the sync process is actually executed. Things to check for would include:

• verification that the working directory is in a git repository
• initial connectivity check (check that we can connect to the remote repository)

We would also like more information about failures to be available through the error messages. These include:

• reporting what caused the sync failure (ie. commit conflicts/connectivity issues)
• reporting which files have conflicts in the case of a conflict