fix: catch standalone command exception

gevent will not be installed by default, therefore print a message
explaining required steps

src/httpaste/__main__.py

@@ -30,7 +30,14 @@ def command_standalone(**kwargs):
     """
 
     from httpaste import load_config, get_flask_app
+
+    try:
         from gevent.pywsgi import WSGIServer
+    except ImportError as e:
+        raise ImportError(' '.join((
+            'gevent is currently not installed.',
+            'Please install it by running \'python3 -m pip install gevent\'.'
+        ))) from e
 
     config, server_config = load_config(kwargs.get('config_path'))