SQLCipher database access errors can raise Soledad exceptions. Database access
and multithreading resources are allocated in different places, so we have to
be careful to close all multithreading mechanismis in case of database access
errors. If we don't, zombie threads may haunt the reactor.

This commit adds SQLCipher exception trapping and Soledad exception raising
for database access errors, while properly shutting down multithreading
resources.