fix(gatsby-plugin-sharp): bail early if sharp isn't working (#10677)

We've seen several issues where builds break because `gatsby-plugin-sharp` fails for some reason. 

Typically because `sharp` didn't build correctly during install like in #10620 most likely due to an incorrect version of python

From https://github.com/nodejs/node-gyp
> python (v2.7 recommended, v3.x.x is not supported)

This PR adds a try-catch around requiring `sharp` and bails early if it fails

Author: sidharthachatterjee

packages/gatsby-plugin-sharp/src/index.js

@@ -1,3 +1,22 @@
+try {
+  require(`sharp`)
+} catch (error) {
+  // Bail early if sharp isn't available
+  console.error(
+    `
+      The dependency "sharp" does not seem to have been built or installed correctly.
+
+      - Try to reinstall packages and look for errors during installation
+      - Consult "sharp" installation page at http://sharp.pixelplumbing.com/en/stable/install/
+      
+      If neither of the above work, please open an issue in https://github.com/gatsbyjs/gatsby/issues
+    `
+  )
+  console.log()
+  console.error(error)
+  process.exit(1)
+}
+
 const sharp = require(`sharp`)
 const crypto = require(`crypto`)
 const imageSize = require(`probe-image-size`)