fix(json): improve JSON parse error reporting

It's not uncommon for services to send back bad JSON on errors, specially things like proxies, or if you get the wrong URI. This way, we can actually find out what the bad JSON was.

package.json

@@ -69,6 +69,7 @@
   },
   "dependencies": {
     "encoding": "^0.1.11",
+    "json-parse-helpfulerror": "^1.0.3",
     "safe-buffer": "^5.0.1"
   }
 }

src/body.js

@@ -11,6 +11,7 @@ const Buffer = require('safe-buffer').Buffer
 const Blob = require('./blob.js')
 const BUFFER = Blob.BUFFER
 const convert = require('encoding').convert
+const parseJson = require('json-parse-helpfulerror').parse
 const FetchError = require('./fetch-error.js')
 const Stream = require('stream')
 
@@ -92,7 +93,7 @@ Body.prototype = {
    * @return  Promise
    */
   json () {
-    return consumeBody.call(this).then(buffer => JSON.parse(buffer.toString()))
+    return consumeBody.call(this).then(buffer => parseJson(buffer.toString()))
   },
 
   /**