Adding a check to verify path strings in front matter are prepended w…

…ith a forward slash

lib/utils/glob-pages.js

@@ -6,6 +6,7 @@ import fs from 'fs'
 import frontMatter from 'front-matter'
 import htmlFrontMatter from 'html-frontmatter'
 import objectAssign from 'object-assign'
+import invariant from 'invariant'
 const debug = require('debug')('gatsby:glob')
 let rewritePath
 try {
@@ -76,6 +77,11 @@ module.exports = (directory, callback) => {
       if (!(parsed.name.slice(0, 1) === '_')) {
         if (data.path) {
           // Path was hardcoded.
+          const pathInvariantMessage = `
+          You're setting path to ${data.path} for ${parsed.dirname},
+          but it should be /${data.path}
+          `
+          invariant((data.path.charAt(0) === '/'), pathInvariantMessage)
           pageData.path = data.path
         } else if (rewritePath) {
           pageData.path = rewritePath(parsed, pageData)