[gatsby-source-filesystem] Add more error handling (#4129)
[gatsby-source-filesystem] Add more error handling (#4129)

* add more error handling in gatsby-source-filesystem plugin

* change error message

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