[lldb-server] Report launch error in vRun packets

Uses our existing "error string" extension to provide a better
indication of why the launch failed (the client does not make use of the
error yet).

Also, fix the way we obtain the launch error message (make sure we read
the whole message, and skip trailing garbage), and reduce the size of
TestLldbGdbServer by splitting some tests into a separate file.

Differential Revision: https://reviews.llvm.org/D133352