Guard against non-existent plugins dir.

piku.py

@@ -1386,13 +1386,14 @@ def _get_plugin_commands(path):
     sys_path.append(abspath(path))
 
     cli_commands = []
-    for item in listdir(path):
-        module_path = join(path, item)
-        if not isdir(module_path):
-            continue
-        module = import_module(item)
-        if hasattr(module, 'cli_commands'):
-            cli_commands.append(module.cli_commands())
+    if isdir(path):
+        for item in listdir(path):
+            module_path = join(path, item)
+            if not isdir(module_path):
+                continue
+            module = import_module(item)
+            if hasattr(module, 'cli_commands'):
+                cli_commands.append(module.cli_commands())
 
     return cli_commands