Revert "[lldb-mi] Add overload method for setting an error"

Summary: This reverts commit r334245 because it duplicates
functionality of Status::AsCString used in SBError.

Reviewers: aprantl, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits, ki.stfu

Differential Revision: https://reviews.llvm.org/D48212

llvm-svn: 334860