ci: Stringify automated registration errors

  When `cozy-stack` returns an error response to an OAuth authorization
  request, we throw a Javascript error with the response body.

  However, when the body is a JSON object, we only get its type in the
  error message rather than the content itself.
  By stringifying the body, we make sure its content will be included in
  the error message and thus displayed in the logs or console.

dev/remote/automated_registration.js

@@ -131,7 +131,9 @@ const authorize = async authorizeUrl => {
   if (redirectUrl) {
     return redirectUrl
   } else {
-    throw new Error(`Authorization failed (code ${res.status}):\n  ${body}`)
+    throw new Error(
+      `Authorization failed (code ${res.status}):\n  ${JSON.stringify(body)}`
+    )
   }
 }