How to show success message and error message in LWC?

1. First import ShowToastEvent 

import { ShowToastEvent } from 'lightning/platformShowToastEvent';

2. Add below method.

showSuccess(){

        this.dispatchEvent(new ShowToastEvent({

          title: 'Success',

          message: 'Lead has been Cloned',

          variant: 'success',

        }));

    }

3. Call this.showSuccess(); where we want to show success message.

4. Error message

showError(msg){

        const evt = new ShowToastEvent({

            title: 'Eligibility Error',

            message: msg,

            variant: "error"

        });

        this.dispatchEvent(evt);

    }

5. Call from catch block to show error message

 .catch(err => {

                    console.log('error', err);

                    this.loading = false;

                    this.showError(err.body.message);

                });

if needed we can show generic error message instead of technical

error message.