Catch exceptions in database init (for example, DB schema errors) (#1722)
Catch exceptions in database init (for example, DB schema errors) (#1722)

Before this commit, if an exception happened in the Database __init__ call,
then the database thread would exit, and parsl would hang at shutdown.

After this commit, exceptions in database creation are handling with the
existing database exception queue.
Parsl/parsl,Parsl/parsl,Parsl/parsl,Parsl/parsl