[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