Improve SBThread's stepping API using SBError parameter.

Summary: The new methods will allow to get error messages from stepping API.

Reviewers: aprantl, clayborg, labath, jingham

Reviewed By: aprantl, clayborg, jingham

Subscribers: apolyakov, labath, jingham, clayborg, lemo, lldb-commits

Differential Revision: https://reviews.llvm.org/D47991

llvm-svn: 335180