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/getchar.c b/src/getchar.c
index 149bf55..39aee22 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -1447,7 +1447,7 @@
 {
     if (curscript + 1 == NSCRIPT)
     {
-	emsg(_(e_nesting));
+	emsg(_(e_scripts_nested_too_deep));
 	return;
     }