Add a new SBTarget::LoadCore() overload which surfaces errors if the load fails

There was no way to find out what's wrong if SBProcess SBTarget::LoadCore(const char *core_file) failed. 
Additionally, the implementation was unconditionally setting sb_process, so it wasn't even possible to check if the return SBProcess is valid.

This change adds a new overload which surfaces the errors and also returns a valid SBProcess only if the core load succeeds:

SBProcess SBTarget::LoadCore(const char *core_file, SBError &error);

Differential Revision: https://reviews.llvm.org/D48049

llvm-svn: 334439