Check if source folder exists and exit with helpful error message

[egil]

Describe the solution you'd like
Inspired by #308, where a previous step in a workflow didn't error out, even if the source folder was not create, a less helpful error message was returned by rsync.

The action could help by checking if the source folder exists before calling rsync, and provide a helpful error message instead, if it doesn't.

[JamesIves]

Released with 3.5.4: https://github.com/JamesIves/github-pages-deploy-action/releases/tag/3.5.4

[JamesIves]

Sadly this was causing some issues so I've had to roll these changes back until we can investigate further. For some reason the fs.existsSync command was causing the action to exit early right after it creates a new branch. Probably due to an error that ended up throwing and catching silently.

This is the commit with the code changes that were rolled back: 5e3a037

I'm guessing the existsSync command is being executed incorrectly.

[JamesIves]

If anyone would like to pick this up for Hacktoberfest please take a look at my previous comment! I didn't have time to investigate why this is happening but it would be a great thing to add!

[JamesIves]

This has been added as part of version 3.7.1.