refactor: report <filename:line:errmsg> for compile errors.

* specs/specl_spec.yaml: New examples for Lua code syntax errors
at various positions within the spec file.
* lib/specl/loader.lua (compile): Move from here...
(parser_mt:compile): ...here. Use the filename for error messages,
rather than spurious `location` parameter.
Adjust all callers.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
refactor: consolidate error reporting from Lua compilation.

* lib/specl/loader.lua (parser_mt:compile): Always report error
messages to standard error instead of raising an error. Simplify
accordingly.
* NEWS: Update.

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