In you action class you will add your error key along with request object like below,
SessionErrors.add(actionRequest, "your-error");
In your JSP you will add:
<%@ taglib uri="http://liferay.com/tld/ui" prefix="liferay-ui" %>
<liferay-ui:error key="your-error" message="Your error message goes here!" />
Also it could be handled through exception class, With SessionErrors code like,
try {
// ... Code which will throws the exception
} catch(SomeException se) {
SessionErrors.add(actionRequest, se.getClass().getName());
}
In jsp
<liferay-ui:error exception="<%= SomeException.class %>" message="This is your Error" />
Reference link: http://www.liferay.com/community/wiki/-/wiki/Main/User+Feedback+-+Success+and+Error+Messages+in+Portlets