error on missing base repo config #6729

With the advent of gh repo default ( #4859 ) we want to stop offering interactive selection of a base repo during actions like creating pull requests and issues. This issue covers modifying the BaseRepo function to error if there are multiple git remote candidates for gh to use as a base repository and none have been explicitly selected yet.

The error should tell users to run gh repo default to select a base repo, then go back to what they were doing.