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/evalfunc.c b/src/evalfunc.c
index d3b2cde..6c467d8 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -2923,7 +2923,7 @@
     if (s == NULL || eval1(&s, rettv, &EVALARG_EVALUATE) == FAIL)
     {
 	if (p != NULL && !aborting())
-	    semsg(_(e_invexpr2), p);
+	    semsg(_(e_invalid_expression_str), p);
 	need_clr_eos = FALSE;
 	rettv->v_type = VAR_NUMBER;
 	rettv->vval.v_number = 0;