Replace convertTo/stringify with non-throwing alternatives.

Having these throw is terrible for ergonomics and there are loads
of places where the exception was either silently ignored or not
handled at all. Having a function which returns an optional and
another that returns a default works a lot better imo.

modules/hostserv/hs_request.cpp

@@ -320,7 +320,7 @@ public:
 				++display_counter;
 
 				ListFormatter::ListEntry entry;
-				entry["Number"] = stringify(display_counter);
+				entry["Number"] = Anope::ToString(display_counter);
 				entry["Nick"] = nick;
 				if (!hr->ident.empty())
 					entry["Vhost"] = hr->ident + "@" + hr->host;