Add runtime error for missing channel factory

Signed-off-by: Ben Sherman <[email protected]>

modules/nextflow/src/main/groovy/nextflow/plugin/extension/PluginExtensionProvider.groovy

@@ -286,6 +286,9 @@ class PluginExtensionProvider implements ExtensionProvider {
             def factory = (ChannelFactoryInstance)reference.target
             return factory.invokeExtensionMethod(reference.method, args)
         }
+        else {
+            throw new MissingMethodException("Channel.${name}", Object.class, args)
+        }
     }
 
 }