yaml: don't throw OOM on any error in yaml encoding

OOM is not single possible error. Yaml library during dump can
raise such errors as YAML_MEMORY_ERROR, YAML_WRITER_ERROR,
YAML_EMITTER_ERROR. And each of them can contain any error
message that is skipped now, because Tarantool YAML does not
provide such API, that can lead to a non-OOM error.

But it is changed in next commits.