[debuginfo-tests][Dexter] Fix some Windows-unfriendly Dexter behaviours
These are some minor things that I've run into on Windows, largely in
error handling paths:
- Giving --lldb-executable on Windows triggers a "useless option" code path, which touches an attribute that only exists in the list_debuggers tool. Switch this to use hasattr, which will work in all subtools.
- We were over-decoding some text reporting errors, but only in an exception path
- The path to lldb on Windows needs to be quoted (even though dexter isn't making use of it).
Differential Revision: https://reviews.llvm.org/D74546