base: ensure export errors report callsite in stack trace.

* specs/base_spec.yaml (export): Finish and simplify mkstack().
Specify callsite line numbers in export errors.
* lib/std/base.lua (formaterror): Allow expectedtypes to be a
string.
(export): Use it to generate error messages.
(export): Set levels correctly for correct callsite reporting.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>