no-process-env: support custom error message "use the env wrapper"

[devinrhode2]

This is the first time I've ever touched eslint rule stuff, but I feel pretty good about this change, and was able to maintain the 100% test coverage on this rule.

[voxpelli]

eslint-plugin-n is the maintained version of this module.

We switched to it in eg. eslint-config-standard / standard and it is maintained by me and other members of the official ESLint community organization.

Try that module out and if it isn't fixed there, then open a new PR in that project.