DPL-402 Improve robustness of Pacbio::LibraryFactory error handling

[JamesGlover]

Describe the Housekeeping
When making some other changes, Rubocop complained about:

spec/models/pacbio/library_factory_spec.rb:119:11: C: RSpec/SubjectStub: Do not stub methods of the object under test.
          allow(factory).to receive(:valid?).and_return true

This test appears to be designed to check for a belt and bracers approach, and ensure that if the initial factory validation fails to catch something, then problems downstream will not result in an inconsistent state. However:

1. It doesn't check the scenario where the container associations fail to persist. In this case it appears that the library will still be persisted.
2. It is a very artificially engineered test, it would make more sense to mock the return values from the saves of each of library and container_material in turn, and check that the other wasn't saved - a test that I believe would fail with the current code.

As a result I think a large re-evaluation of the error handling should be considered. I would suggest:

1. Consider switching the library.save && @container_material.save to use save! instead. We're generally expecting the earlier validation to catch everything, so this should indeed be 'exceptional'
2. If the saves fail, currently the errors will probably be eaten, save will return false, but errors will still be blank, following the original save. This could be quite frustrating.

The test also doesn't consider other, perhaps more likely, error scenarios, such as database connection troubles, or index constraint violations. We should expect the factory to be transactional, and would need to evaluate the best way of handling these errors. (Ie. What is appropriate to return to the users, what should we be notified about)

Blocking issues
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Additional context
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