emitter: propagate errors correctly.

* specs/ext_yaml_emitter_spec.yaml: Specify error messages are
propagated correctly.
* ext/yaml/emitter.c (emit_STREAM_START, emit_MAPPING_START)
(emit_SEQUENCE_START, emit_SCALAR): Push error messages onto the
errbuff stack.
* NEWS: Update.

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