[lldb] Set result error state in 'frame variable'

Ensure that errors in `frame variable` are reflected in result object.

The statistics for `frame variable` show invocations as being successful, even
when executing one of the error paths.

This change replaces `result.GetErrorStream()` with `result.AppendError()`,
which also sets the status to `eReturnStatusFailed`.

Differential Revision: https://reviews.llvm.org/D116788