add an explicit error

Author: Laurie Barth

packages/gatsby-plugin-theme-ui/gatsby-node.js

@@ -1,8 +1,25 @@
 exports.onPreInit = (__, options) => {
     let {themeModulePath} = options
+    
     if(themeModulePath) {
+      try {
         options.themeModulePath = require(themeModulePath)
-    }
+      } catch {
+        reporter.error(`It appears your theme dependency is not installed. Try running \`${generateInstallInstructions()} ${themeModulePath}\``)
+      }
+  }
+}
+
+
+function generateInstallInstructions() {
+  const { getConfigStore } = require(`gatsby-core-utils`)
+
+  const packageMangerConfigKey = `cli.packageManager`
+  const PACKAGE_MANGER = getConfigStore().get(packageMangerConfigKey) || `yarn`
+
+  const installKeyWord = PACKAGE_MANGER === `yarn` ? "add" : "install"
+
+  return `${PACKAGE_MANGER} ${installKeyWord}`
 }
 
 exports.createSchemaCustomization = ({ actions }) => {

packages/gatsby-plugin-theme-ui/package.json

@@ -6,7 +6,8 @@
   "license": "MIT",
   "peerDependencies": {
     "gatsby": "^2.13.1",
-    "theme-ui": "^0.3.0"
+    "theme-ui": "^0.3.0",
+    "gatsby-core-utils": "^1.2.0"
   },
   "keywords": [
     "gatsby",