catch proper ExecExceptions

README.md

@@ -22,6 +22,8 @@
 
 The conspicuously missing config.php looks like:
 
+(you'll need to create the `WORKING_DIR`)
+
 ```php
 <?php
 

builder.php

@@ -93,7 +93,7 @@ function array_intersperse($array, $val) {
 function outrunner($msg) {
     try {
         return runner($msg);
-    } catch (ExecException $e) {
+    } catch (\GHE\ExecException $e) {
         var_dump($e->getMessage());
         var_dump($e->getCode());
         var_dump($e->getOutput());