Added an optional file with MacErrors.h extensions: IC errors aren't in there.

Bugfix candidate.
diff --git a/Mac/scripts/mkestrres.py b/Mac/scripts/mkestrres.py
index c5ff6d3..bf56e43 100644
--- a/Mac/scripts/mkestrres.py
+++ b/Mac/scripts/mkestrres.py
@@ -123,6 +123,12 @@
 	parse_errors_h(fp, dict)
 	fp.close()
 	
+	fss, ok = macfs.PromptGetFile("Where is mkestrres-MacErrors.h?")
+	if not ok: return
+	fp = open(fss.as_pathname())
+	parse_errors_h(fp, dict)
+	fp.close()
+	
 	if not dict:
 		return