This change removes unused exceptions defined in core/errors.py,
along with the errors.py module itself. The few used items from
this file are migrated over to core/exceptions.py. Any dependent
code is updated, dropping the use of errors.py and adding in the
use of exceptions.py.
This change reorganizes the test suite, splitting it into two separate
test suites, one for unit tests and one for integration tests. The tox
configuration is also updated to reflect this change. Invoking tox now
runs the unit tests by default. A separate tox environment is used to
run the integration tests.
