Salesforce custom JavaScript button example that updates record and displays errors.

JavaScriptButton.js

{!REQUIRESCRIPT("/soap/ajax/27.0/connection.js")}

try {

  var formToUpdate = new sforce.SObject( "Company_Form_Opp__c" );

  formToUpdate.id = "{!Company_Form_Opp__c.Id}";

  // ... snip ...

  var result = sforce.connection.update( [ formToUpdate ] );

  if ( result[0].getBoolean( "success" ) ) {

    location.reload( true ); // refresh page

  } else {

    var errors = result[0].errors;
    var errorMessages = "";

    for ( var i = 0; i < errors.length; i++ ) {
      errorMessages += errors[i].message + '\n';
    }

    alert( errorMessages ); // display all validation errors

  }

} catch ( ex ) {

  alert( ex ); // display any javascript exception message

}