Google App Engine probits the creation of new threads. This leads to a
failure in BackgroundPreinitializer when the single thread executor
attempts to create its single thread.

This commit enhances the existing fail safety of
BackgroundPreinitializer by catching any exceptions thrown while
creating the executor and submitting the tasks to it. Any initialisation
that has not performed in the background will be performed in the
foreground instead.

Closes gh-4662