fix(forms): export FORM_ERROR

Author: zouxuoz

packages/forms/src/index.ts

@@ -4,3 +4,4 @@ export { Fieldset } from './Fieldset';
 export { FieldArray } from './FieldArray';
 export { FormContext } from './FormContext';
 export { FormSpy } from 'react-final-form';
+export { FORM_ERROR } from 'final-form';