patch 8.2.3069: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move some error messages to errors.h.  Use clearer names.
diff --git a/src/ex_getln.c b/src/ex_getln.c
index e7d71a5..22f1cf6 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -2622,7 +2622,7 @@
 {
 #ifdef FEAT_CMDWIN
     if (cmdwin_type != 0)
-	return e_cmdwin;
+	return e_invalid_in_cmdline_window;
 #endif
     if (textwinlock != 0)
 	return e_textwinlock;