Customize error message for trigger and display error message below the field

This Post demonstrates how to display error message in particular field level.

SObject SomeRecord;

SomeRecord.SomeField__c.addError('Custom Message');

//or

SomeRecord.someField__c.addError(someException);

Note:

We cannot add it to a field dynamically. This error mapping can only be done with a hard-coded field.

Useful Links:
https://salesforce.stackexchange.com/questions/79291/how-to-use-adderror-with-a-dynamically-derived-field-name

List has no rows for assignment to SObject  error

 Consider following query:

   
      id p;

      user u = [SELECT Id from user where Name = :username];

      if (u != null)

      p = u.Id;

•  The above code will fail if there is no user record with the matching username. It doesn't actually return a null.

 It would be safer to do the following:

    id p;

     user[] userlist = [SELECT Id from user where Name = :username];

     if (userlist.size() > 0)

       p = userlist[0].Id;