Arbitrarily large output can't be used in an exception.

lib/cros_build_lib.py

Index: lib/cros_build_lib.py
diff --git a/lib/cros_build_lib.py b/lib/cros_build_lib.py
index 804e915aa62c7cfe058f12c532ee4a928cd77ec7..5a1c0d2e770cad45343e86c8549a2deb2bde559e 100644
--- a/lib/cros_build_lib.py
+++ b/lib/cros_build_lib.py
@@ -110,8 +110,12 @@ def RunCommand(cmd, print_cmd=True, error_ok=False, error_message=None,
 
   # If the command (and all retries) failed, handle error result
   if proc.returncode != 0 and not error_ok:
+    if output:
+      print >> sys.stderr, output
+      sys.stderr.flush()
+
     error_info = ('Command "%r" failed.\n' % (cmd) +
-                  (error_message or error or output or ''))
+                  (error_message or error or ''))
     if log_to_file: error_info += '\nOutput logged to %s' % log_to_file
     raise RunCommandException(error_info)