Provide better error message if Docker is not running

[philwebb]

The current error message could be improved

$ mvn spring-boot:build-image

...
Execution default-cli of goal org.springframework.boot:spring-boot-maven-plugin:2.3.0.RELEASE:build-image failed: Docker API call to 'localhost/v1.24/images/create?fromImage=gcr.io%2Fpaketo-buildpacks%2Fbuilder%3Abase-platform-api-0.3' failed with status code 500 "com.sun.jna.LastErrorException: [2] No such file or directory"

[philwebb]

This is related to #21515