Improved error reporting when deserializing a non-existent record

**DelayedJob** raises an overly generic `Delayed::DeserializationError` when it tries to load a serialized record that does not exist in the DB.
This situation can come to be because of a serialization error, or because the record has been deleted since the job was enqueued.

The error is raised because the default **YAML** behaviour of loading an object is [patched](https://github.com/collectiveidea/delayed_job/blob/master/lib/delayed/psych_ext.rb#L39) to call `klass.find(id)` if the serialized object comes from a recognized persistence library (e.g. **ActiveRecord**).

There have already been discussions about this behaviour: [#231](https://github.com/collectiveidea/delayed_job/pull/231), [#282](https://github.com/collectiveidea/delayed_job/pull/282) and [#296](https://github.com/collectiveidea/delayed_job/pull/296).
It seems like the **DJ** leads have a strong opinion on this design decision, and no patch has been merged in yet.

I'm not here to argue with the current behaviour and its implications and side effects, but I think we can do something about the error messages to make it clearer what is going on. At the moment **DJ** is catching the correct errors (e.g. `ActiveRecord::RecordNotFound`), but it's also completely discarding any information that comes with them.

This PR adds more descriptive error messages, borrowing from what has already been done [here](https://github.com/collectiveidea/delayed_job/blob/master/lib/delayed/serialization/active_record.rb#L9).
I did (in the initial commit), but rubocop flagged it. It expressly suggested the `raise(Error.new("msg")` syntax.

I see that it's also used elsewhere in the project.

The other warnings from rubocop are not caused by changes introduced in this PR.