Improve bin/detect error messages

[jmorrell]

Before:

       Node.js: package.json not found in application root

After:

!     ERROR: 'package.json' listed in '.slugignore' file
!
!     The 'heroku/nodejs' buildpack is set on this application, but was
!     unable to detect a 'package.json' file. This is likely because
!     the '.slugignore' file is removing it before the build begins.
!
!     For more information, refer to the following documentation:
!     https://devcenter.heroku.com/articles/slug-compiler#ignoring-files-with-slugignore

!     ERROR: 'package.json' listed in '.gitignore' file
! 
!     The 'heroku/nodejs' buildpack is set on this application, but was
!     unable to detect a 'package.json' file. This is likely because
!     the '.gitignore' file is preventing it from being checked in to
!     the git repo.

!     For more information, refer to the following documentation:
!     https://devcenter.heroku.com/articles/gitignore

 !     ERROR: Application not supported by 'heroku/nodejs' buildpack
 !
 !     The 'heroku/nodejs' buildpack is set on this application, but was
 !     unable to detect a Node.js codebase.
 !
 !     A Node.js app on Heroku requires a 'package.json' at the root of
 !     the directory structure.
 !
 !     If you are trying to deploy a Node.js application, ensure that this
 !     file is present at the top level directory. This directory has the
 !     following files:
 !
 !     LICENSE
 !     README.md
 !     index.js
 !     src/
 !
 !     If you are trying to deploy an application written in another
 !     language, you need to change the list of buildpacks set on your
 !     Heroku app using the 'heroku buildpacks' command.
 !
 !     For more information, refer to the following documentation:
 !     https://devcenter.heroku.com/articles/buildpacks
 !     https://devcenter.heroku.com/articles/nodejs-support#activation

[CaseyFaist]

I love the new messages! Future refactor could be to extract error text and pull in using env vars or cating to separate error detection and documentation, but not necessary for this update. 💯 for tests, too 👍