how to write Error message


I wanted to perform functional tests on the input fields (user-entered information) in the configuration window, where I have to do these tests and display the verification messages




You do these in the NodeMode#configure method and if the configuration isn't valid you can throw an new InvalidSettingsException.

The message in this setting will be put into the console and will be viewable when you hover over the error icon on the node. 

You could also add checks to display to the user in the NodeDialog class.