[lldb] Print the exception traceback when hitting cleanup errors

Right now if the test suite encounters a cleanup error it just prints "CLEANUP
ERROR:" but not any additional information.

This patch just prints the exception that caused the cleanup error. This should
make debugging the failing tests for D83865 easier (and seems in general nice to
have).

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D83874