appflow update & appflow checkin commands seem fragile

In the past two weeks I had the following scenario twice:

1. `appflow update` works and does the updates, which is fine
2. `appflow decrypt` works, so I change a few values
3. `appflow checkin ` seems to work from the messages in terminal, but nothing gets checked in

So naturally I go to `cd ~/.appflow/tenant//` and see that it has unpushed commits and another user checked in something and I have a ton of merge conflicts after `git pull`.

Manually fixing it and redoing my work using:

`git fetch origin master`
`git reset --hard origin/master`

After this it works again with the commands.

## Suggestion

Add better checks to `appflow update` and better messages if it couldn't push the changes or has merge conflicts for `appflow checkin`.