fix: parse JSON from GitHub error messages

BREAKING CHANGE: error.message is no longer a JSON string containing GitHub’s error message. Instead the JSON is parsed internally and error properties are assigned with its value. This is not technically a breaking change but a bug fix, but as this bug was around for a long time and plenty of apps depend on `JSON.parse(error.message)` to be working, we treat it as a breaking change by way of precaution

lib/request/request.js

@@ -99,6 +99,12 @@ function request (requestOptions) {
 
     .catch(error => {
       if (error instanceof HttpError) {
+        try {
+          Object.assign(error, JSON.parse(error.message))
+        } catch (_error) {
+          // ignore, see #684
+        }
+
         throw error
       }