Added an exception when the scm command fails.

This will help the debugging for people.

src/IDF/Scm.php

@@ -55,6 +55,9 @@ class IDF_Scm
         $cache = Pluf_Cache::factory();
         if (null === ($res=$cache->get($key))) {
             $ll = exec($command, $output, $return);
+            if ($return != 0) {
+                throw new IDF_Scm_Exception(sprintf('Error when running command: "%s", return code: %d', $command, $return));
+            }
             $cache->set($key, array($ll, $return, $output));
         } else {
             list($ll, $return, $output) = $res;