Provide meaningful error message when branch to merge from is not available to merge

[shrink]

Per #5, it's not obvious that fetch-depth: 0 is required and when it isn't included it's not obvious that it is the source of the error.

• Add a step to verify that the branch is available to merge, and provide a helpful error message when it is not
• Add documentation describing the need for fetch-depth from the example
• Replace incorrect use of source with correct use of from in README introduction