You can add an error to a specific field using the `addError` method as follows:

    SObject someRecord;
    someRecord.SomeField__c.addError('Custom Message');
    //or
    someRecord.someField__c.addError(someException);

Note that you cannot add it to a field dynamically. This error mapping can only be done with a hard-coded field. See also:

- [Provide an additional addError method on SObject that accepts a field name](https://success.salesforce.com/ideaview?id=08730000000GoC5AAK)
- http://salesforce.stackexchange.com/questions/79291/how-to-use-adderror-with-a-dynamically-derived-field-name