Client: clear validation errors on dapi fieldLoaded

Previously, the system relied on the preStagingUpdate StagingBucket
event to do this implicitly, but that is no longer kicked off when
the diff doesn't produce any bucket changes.

* src/client/Client.js (_createProgram) [dapi]: Clear validation
  failures on dapi fieldLoaded.

src/client/Client.js

@@ -1147,6 +1147,12 @@ module.exports = Class( 'Client' )
                 _self.getUi().getCurrentStep().getElementGroup( name )
                     .clearOptions( name, index );
             } )
+            .on( 'fieldLoaded', ( name, index ) =>
+            {
+                _self._dataValidator.clearFailures( {
+                    [name]: [ index ],
+                } );
+            } )
             .on( 'error', function( e )
             {
                 _self._handleError( e );