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/chanserv/cs_flags.cpp

@@ -331,7 +331,7 @@ class CommandCSFlags final
 
 			ListFormatter::ListEntry entry;
 			++count;
-			entry["Number"] = stringify(i + 1);
+			entry["Number"] = Anope::ToString(i + 1);
 			entry["Mask"] = access->Mask();
 			entry["Flags"] = flags;
 			entry["Creator"] = access->creator;