patch 8.2.4492: no error if an option is given a value with ":let &opt = val"

Problem:    No error if an option is given an invalid value with
            ":let &opt = val".
Solution:   Give the error. (closes #9864)
diff --git a/src/evalvars.c b/src/evalvars.c
index 862d519..eb69d3e 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -1518,8 +1518,11 @@
 	{
 	    if (opt_type != gov_string || s != NULL)
 	    {
-		set_option_value(arg, n, s, scope);
+		char *err = set_option_value(arg, n, s, scope);
+
 		arg_end = p;
+		if (err != NULL)
+		    emsg(_(err));
 	    }
 	    else
 		emsg(_(e_string_required));