refactor: report errors and exit consistently.

* specs/specl_spec.yaml: Add specifications for libYAML parser
error output.
* lib/specl/loader.lua (parser_mt:error): Don't raise an error
and force a backtrace, just report the error and position to
stderr then exit gracefully.
(parser_mt:parse): Capture errors raised by libYAML and handle
them with parser_mt:error, like everything else.
* NEWS: Update.

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