Improve replicate error handling tests

Re-implement test for get failure and add a test for bulkDocs failure.

Make the tests independent of the internals of the replication algorithm.
In particular, independent of the checkpoint. Rather, test the external
results of a failed replication followed by a successful replication.
Improve error on replication abort

Modeled after the errors in error. Include the replicate context of the error
and whatever error was received by replicate.