avocado.core.plugins.builtin: Return warnings on stderr

In order to avoid confusion among users as to whether
a plugin is disabled or not, enforce that through outputting
warnings on the app stderr.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/core/plugins/builtin.py

@@ -52,6 +52,7 @@ def load_builtins():
         except Exception as err:
             name = str(module)
             reason = '%s %s' % (str(err.__class__.__name__), err)
+            log.error('Error loading %s -> %s', name, reason)
             ErrorsLoading.append((name, reason))
             continue
         for name in plugin_mod.__dict__: