IOS-631 Fix crash when Publication::baseURL is nil

[ettore]

What's this do?

• fixes a crash happening because Readium has a precondition statement in an initializer of an internal class (EPUBNavigatorViewModel)
• fixing the crash involves not opening the reader and presenting an error message. This caused a spinner on the "Read" button to never stop spinning. The 2nd commit addresses this UX problem by converting a successCompletion handler into a regular completion(_:Bool) handler called for failure cases too.

Why are we doing this? (w/ JIRA link if applicable)
IOS-631

How should this be tested? / Do these changes have associated tests?
We never determined a reproducible case for this crash. I'd say we should verify book opening does not present any regressions.

Dependencies for merging? Releasing to production?
this is dependent on #1628
Once that is merged I will change the base branch to develop.

Does this include changes that require a new SimplyE/Open eBooks build for QA?
not yet

Has the application documentation been updated for these changes?
yes

Did someone actually run this code to verify it works?
I did