[lldb] Remove redundant calls to set eReturnStatusFailed
Since https://reviews.llvm.org/D103701 AppendError<...>
sets this for you.
This change includes all of the non-command uses.
Some uses remain where it's either tricky to reason about
the logic, or they aren't paired with AppendError calls.
Reviewed By: teemperor
Differential Revision: https://reviews.llvm.org/D104379