Validate when creating a page that the page component exists (#3997)
Validate when creating a page that the page component exists (#3997)

* Validate when creating a page that the page component exists

* Don't check for component paths during testing as it breaks tests

ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,fk/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,0x80/gatsby,gatsbyjs/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,mingaldrichgan/gatsby,mingaldrichgan/gatsby,0x80/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby