Add descriptive warning for invalid JSON in package.json

[jmorrell]

Of the builds that fail to deploy on Heroku, between 5-10% of them fail because package.json is not valid JSON.

This error currently looks like this:

invalid-package-json git/master  
❯ git push heroku master
Counting objects: 72, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (33/33), done.
Writing objects: 100% (72/72), 42.49 KiB | 0 bytes/s, done.
Total 72 (delta 31), reused 64 (delta 29)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote: -----> Node.js app detected
remote: parse error: Expected separator between values at line 9, column 14
remote:  !     Unable to parse package.json
remote: 
remote: 
remote: -----> Build failed
remote: parse error: Expected separator between values at line 9, column 14
remote: parse error: Expected separator between values at line 9, column 14
remote:        
remote:        We're sorry this build is failing! You can troubleshoot common issues here:
remote:        https://devcenter.heroku.com/articles/troubleshooting-node-deploys
remote:        
remote:        If you're stuck, please submit a ticket so we can help:
remote:        https://help.heroku.com/
remote:        
remote:        Love,
remote:        Heroku
remote:        
remote:  !     Push rejected, failed to compile Node.js app.
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !       Push rejected to sheltered-hollows-52627.
remote: 
To https://git.heroku.com/sheltered-hollows-52627.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/sheltered-hollows-52627.git'

We can do much better than this.