test coverage increase for core

[Tlacenka]

In this issue, all coverage related subtasks from the test refactor are present.

• [ ] Add a happy path test to json-to-gql. Consider moving this file to utils.
• [ ] Add a test case for the no commit data available case in persist if not too difficult.
• [x] The e2e test (snapshot) caught a additional property runner that slipped through when refactoring executePlugin. This should have been caught by one of the executePlugin tests. #232
• [x] Add test for PersistDirError in persist #220
• [x] Add test for PersistError in persist #221
• [x] Add test and better error for failed plugins. In this example the 3rd plugin failed. It would help to add the plugin slug at the top of the error.