[dotest] Add logging to investigate CI issue.

We're seeing an odd issue on GreenDragon's lldb-cmake-matrix. Dotest is
unable to move a log file (OSError: [Errno 2] No such file or
directory). The os.rename call is guarded with a check that the source
file and destination directory exist.

This wraps the call in a try-except that prints the source and
destination path to see which component seemingly doesn't exist.

llvm-svn: 351611