Added eslint rule to prevent usage of new Error()

- we have our own error library that should always be used to wrap errors in useful info
- therefore instead of new Error() we should always be using errors.SomeError from @tryghost/error

.eslintrc.js

@@ -13,7 +13,12 @@ module.exports = {
         // @TODO: remove this rule once it's turned into "error" in the base plugin
         'no-shadow': 'error',
         'no-var': 'error',
-        'one-var': [2, 'never']
+        'one-var': [2, 'never'],
+        'no-restricted-syntax': ['warn',
+            {
+                selector: 'NewExpression[callee.name=\'Error\']',
+                message: 'Direct calls to new Error() are not allowed. Please use @tryghost/errors.'
+            }]
     },
     overrides: [
         {