Revert "Return a named error in the result object of an expression with no result"

This reverts commit r344647.
This causes build failures with [-Werror, -Wswitch]. Some cases where the newly
introduced enum value is not handled in particular are in:
  lldb/source/Expression/REPL.cpp:350
  lldb/source/Interpreter/CommandInterpreter.cpp:1529
(maybe there could be more)

As I don't understand lldb to make sure the likely trivial fixes are
correct and also as they might need additional tests, leaving to the
author to resolve.

llvm-svn: 344722