Changed handling of logged exceptions

In OXID eShop the business logic sometime requires to only log exceptions and not (re)throw them.
As an result some faulty behavior might be overlooked, if the tests do not evaluate the exception log file.
To improve test quality as of this release, before and after each test the exception log file is evaluated.

This is a breaking change as tests will fail, if the exception log is not empty.

If your tests willingly produce some content in the exception log, you can assert this content with the new method
\OxidEsales\TestingLibrary\BaseTestCase::assertLoggedException()