Fix problematic override _exc_info_to_string
- pass through to base-class implementation when raised exception is not from an LLDBTest
- should make the test suite errors a little easier to root-cause



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@187450 91177308-0d34-0410-b5e6-96231b3b80d8