Fix `ResolveCustomSource()` not always copying argument on failure

sources/Aliases/AliasesAPI.uc

@@ -349,7 +349,11 @@ public final function Text ResolveCustom(
     local BaseAliasSource customSource;
 
     customSource = GetCustomSource(sourceName);
-    if (customSource == none) {
+    if (customSource == none)
+    {
+        if (copyOnFailure && alias != none) {
+            return alias.Copy();
+        }
         return none;
     }
     return ResolveWithSource(alias, customSource, copyOnFailure);