[lldb-mi] Clean up and update a few MI commands.

Summary:
This patch updates a few MI commands using a new way of
handling an errors in lldb-mi and removes unnecessary
m_lldbResult variables.

Reviewers: aprantl, clayborg, labath

Reviewed By: aprantl, clayborg

Subscribers: ki.stfu, lldb-commits

Differential Revision: https://reviews.llvm.org/D47992

llvm-svn: 336009