llgs-tests: Replace the "log+return false" pattern with llvm::Error

Summary:
These tests used to log the error message and return plain bool mainly
because at the time they we written, we did not have a nice way to
assert on llvm::Error values. That is no longer true, so replace this
pattern with a more idiomatic approach.

As a part of this patch, I also move the formatting of
GDBRemoteCommunication::PacketResult values out of the test code, as
that can be useful elsewhere.

Reviewers: zturner, eugene

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D39790

llvm-svn: 317795