Move AIAlert* into namespace AIAlert and rename classes.

Instead of having several AI* classes, it turned out to be easier to
have a namespace: this allows me to define the classes in llcommon but
add (new) 'add' and 'add_modal' functions to 'AIAlert' in llui.
This is needed to avoid a collision with the 'add' functions in
LLNotificationsUtil.

The new add/add_modal makes it a lot easier to just show a caught
alert, prepending or appending new text: it turns out that that is
way more common then a re-throw.

Adjusted code as appropriate.

indra/llui/llnotificationsutil.cpp

@@ -30,11 +30,46 @@
 #include "llsd.h"
 #include "llxmlnode.h"	// apparently needed to call LLNotifications::instance()
 
-LLNotificationPtr LLNotificationsUtil::add(AIAlert const& alert, unsigned int suppress_mask)
+namespace AIAlert
 {
-	return LLNotifications::instance().add(alert, suppress_mask);
+
+LLNotificationPtr add(Error const& error, modal_nt type, unsigned int suppress_mask)
+{
+	return LLNotifications::instance().add(error, type, suppress_mask);
 }
 
+LLNotificationPtr add(std::string const& xml_desc, modal_nt type)
+{
+	return LLNotifications::instance().add(Error(Prefix(), type, xml_desc, AIArgs()), type, 0);
+}
+
+LLNotificationPtr add(std::string const& xml_desc, AIArgs const& args, modal_nt type)
+{
+	return LLNotifications::instance().add(Error(Prefix(), type, xml_desc, args), type, 0);
+}
+
+LLNotificationPtr add(Error const& error, std::string const& xml_desc, unsigned int suppress_mask, modal_nt type)
+{
+	return LLNotifications::instance().add(Error(Prefix(), type, error, xml_desc, AIArgs()), type, suppress_mask);
+}
+
+LLNotificationPtr add(Error const& error, std::string const& xml_desc, AIArgs const& args, unsigned int suppress_mask, modal_nt type)
+{
+	return LLNotifications::instance().add(Error(Prefix(), type, error, xml_desc, args), type, suppress_mask);
+}
+
+LLNotificationPtr add(std::string const& xml_desc, Error const& error, unsigned int suppress_mask, modal_nt type)
+{
+	return LLNotifications::instance().add(Error(Prefix(), type, xml_desc, AIArgs(), error), type, suppress_mask);
+}
+
+LLNotificationPtr add(std::string const& xml_desc, AIArgs const& args, Error const& error, unsigned int suppress_mask, modal_nt type)
+{
+	return LLNotifications::instance().add(Error(Prefix(), type, xml_desc, args, error), type, suppress_mask);
+}
+
+} // namespace AIAlert
+
 LLNotificationPtr LLNotificationsUtil::add(const std::string& name)
 {
 	return LLNotifications::instance().add(