feat: improve error message on missing yaml

mentioned in commit a3edd395

mentioned in merge request to-be-continuous/samples/php-cnb-on-kubernetes!55 (merged)

mentioned in merge request to-be-continuous/samples/golang-cnb-on-kubernetes!64 (merged)

mentioned in merge request to-be-continuous/samples/php-on-kubernetes!114 (merged)

mentioned in merge request to-be-continuous/samples/maven-on-kubernetes!122 (merged)