refactor(index): use TypeError over Error (#166)

index.js

@@ -95,7 +95,7 @@ function chooseAjvInstance (defaultInstance, ajvOpts) {
   } else if (typeof ajvOpts === 'object' && typeof ajvOpts.customOptions === 'function') {
     const ajv = ajvOpts.customOptions(getDefaultInstance())
     if (!(ajv instanceof Ajv)) {
-      throw new Error('customOptions function must return an instance of Ajv')
+      throw new TypeError('customOptions function must return an instance of Ajv')
     }
     return ajv
   }