Bug 1164713 - Return error string from FxAccounts. r=ferjm

services/fxaccounts/FxAccountsManager.jsm

@@ -221,7 +221,7 @@ this.FxAccountsManager = {
         }
       );
     }
-    return Promise.reject(reason);
+    return Promise.reject(reason.message ? { error: reason.message } : reason);
   },
 
   _getAssertion: function(aAudience, aPrincipal) {

services/fxaccounts/tests/xpcshell/test_manager.js

@@ -396,6 +396,7 @@ add_test(function() {
       do_throw("Unexpected success");
     },
     error => {
+      do_check_eq(error.error, ERROR_OFFLINE);
       FxAccountsManager._fxAccounts._reset();
       Services.io.offline = false;
       certExpired = false;