CiviUnitTestCase - Fix edge-case for mis-reported error

CiviUnitTestCase - Fix edge-case for mis-reported error

Suppose you run a test and it encounters an error very early in process --
e.g.  while processing `setUp()`, before constructing the instance of
`$this->hookClass`.  It then proceeds to `tearDown()`. Later, the test
runner will give you a report on the rrors.

Before
------------

The `tearDown` fails because the missing instance of `$this->hookClass`
raises another error, and test-report shows this misleading reference.

After
------------

The `tearDown` proceeds, and the test-report shows the real cause
of the failure.