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/typval.c b/src/typval.c
index 21ecd87..22e9358 100644
--- a/src/typval.c
+++ b/src/typval.c
@@ -1328,7 +1328,7 @@
 		      : STR2NR_ALL, &n, NULL, 0, TRUE);
 	if (len == 0)
 	{
-	    semsg(_(e_invexpr2), *arg);
+	    semsg(_(e_invalid_expression_str), *arg);
 	    return FAIL;
 	}
 	*arg += len;