Clarify the error message reporting that an account ID could not be loaded to indicate the valid sources of an account ID.

lib/actions/EndpointBuildApiGateway.js

@@ -131,7 +131,7 @@ module.exports = function(S) {
 
       // If no iamRoleLambda, throw error
       if (!_this.awsAccountNumber) {
-        throw new SError('No Lambda IAM Role found');
+        throw new SError('Neither an accountId variable nor a Lambda IAM Role variable containing the account ID were found');
       }
 
       // Get populated endpoint