Improve graceful shutdown error handling in api project templates

[jessjenkins]

Look to improve the graceful shutdown code in project_generation/content/templates/api/service/service.go.tmpl

If an error is thrown in previous steps arguably the shutdown was not graceful - I've have refactored my code in dp-observation-importer to handle such cases if you wanted to take a look; it might be ott though

Originally posted by @nshumoogum in #35