Validate when creating a page that the page component exists

Author: KyleAMathews

packages/gatsby/src/redux/actions.js

@@ -122,10 +122,25 @@ actions.createPage = (page: PageInput, plugin?: Plugin, traceId?: string) => {
     noPageOrComponent = true
   }
 
+  if (!fs.existsSync(page.component)) {
+    const message = `${name} created a page with a component that doesn't exist`
+    // Don't log out when testing
+    if (process.env.NODE_ENV !== `test`) {
+      console.log(``)
+      console.log(chalk.bold.red(message))
+      console.log(``)
+      console.log(page)
+    } else {
+      return message
+    }
+    noPageOrComponent = true
+  }
+
   if (!page.component || !path.isAbsolute(page.component)) {
     const message = `${name} must set the absolute path to the page component when create creating a page`
     // Don't log out when testing
     if (process.env.NODE_ENV !== `test`) {
+      console.log(``)
       console.log(chalk.bold.red(message))
       console.log(``)
       console.log(page)
@@ -140,6 +155,7 @@ actions.createPage = (page: PageInput, plugin?: Plugin, traceId?: string) => {
     console.log(
       `See the documentation for createPage https://www.gatsbyjs.org/docs/bound-action-creators/#createPage`
     )
+    console.log(``)
     process.exit(1)
   }