Don’t halt Gulp watch on JS/SASS errors

Problem
---
You make a minor typo, save the file and your Gulp process dies without
you realising. You then spend 5 minutes trying to work out why your
changes aren’t appearing, no matter what edits you make.

Solution
---

Catch errors raised in processing the JS/SASS files and log them,
without killing the process.

How
---

Use a handy plugin called
[Gulp PrettyError](https://www.npmjs.com/package/gulp-prettyerror).

package.json

@@ -41,6 +41,7 @@
   "devDependencies": {
     "gulp-css-url-adjuster": "0.2.3",
     "gulp-jshint": "2.0.0",
+    "gulp-prettyerror": "1.2.1",
     "gulp-sass-lint": "1.1.1",
     "jshint": "2.9.1",
     "jshint-stylish": "2.1.0"