Rescue from SyntaxError when deserializing YAML

If the YAML for the payload object is syntactically incorrect `Psych` will raise a `SyntaxError` which was not captured by `Delayed::Backend::Base::payload_object`. 

The syntax error happend because the payload was truncated during serialisation to the database. Payload data was longer than DB column size.

When the `SyntaxError` was raised, delayed_job quit and no other jobs in the queue were processed.