lyaml: improve parser error handling.

Fix a TODO in the lyaml parser.
* specs/lib_lyaml_spec.yaml (loading): Specify line and column
prefixed error behaviours.
* lib/lyaml.lua (Parser): Initialise line and column.
(parser_mt.error): Raise an error with line and column prefixed.
(parser_mt.parse): Capture libYAML event errors, and forward them
to parser_mt.errer.
Update 1-based line and column markers from 0-based libYAML event
markers.
(parser_mt.loadalias): Raise errors with parser_mt.error.

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