Fix the std::string formatter to report errors in the case where the

string points to unaccessible memory.

The formatter tries to get the data field of the std::string, and to
check whether that fails it just checks that the ValueObjectSP
returned is not empty. But we never return empty ValueObjectSP's to
indicate failure, since doing so would lose the Error object that
tells you why fetching the ValueObject failed.

This patch adds a check for ValueObject::GetError().Success().

I also added a test case for this failure, and reworked the test case
a bit (to use run_to_source_breakpoint). I also renamed a couple of
single letter locals which don't follow the lldb coding conventions.

Differential Revision: https://reviews.llvm.org/D108228