patch 9.0.0359: error message for wrong argument type is not specific

Problem:    Error message for wrong argument type is not specific.
Solution:   Include more information in the error. (Yegappan Lakshmanan,
            closes #11037)
diff --git a/src/blob.c b/src/blob.c
index 0d88c75..cfc3d37 100644
--- a/src/blob.c
+++ b/src/blob.c
@@ -667,11 +667,8 @@
 	initial.vval.v_number = blob_get(b, 0);
 	i = 1;
     }
-    else if (argvars[2].v_type != VAR_NUMBER)
-    {
-	emsg(_(e_number_expected));
+    else if (check_for_number_arg(argvars, 2) == FAIL)
 	return;
-    }
     else
     {
 	initial = argvars[2];