Do not wrap SO errors in Boom

[mshustov]

SavedObjects client wraps ES errors in Boom. https://github.com/elastic/kibana/blob/master/src/core/server/saved_objects/service/lib/decorate_es_error.ts
We already have SavedObjectsErrorHelpers that encapsulates that knowledge. In theory, it should simplify the cleanup, although, we need to inspect solution code to make sure Boom-specific API is not used.

// before
try {
  await client.do.something();
} catch (error) {
  if (error.status === 409) { /* handle */ }
}
// after
try {
  await client.do.something();
} catch (error) {
  if (savedObjectsClient.errors.isConflictError(error)) { /* handle */ }
}

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)