Solve hang on Windows when lldb fails to launch the process.

The DebuggerThread was detecting the launch error, but it was
ignored by ProcessWindows::DoLaunch, causing LLDB to wait forever
in the debugger loop.

This fixes the test case that explicitly attempts to launch a
process from a non-existant path.

Patch by Adrian McCarthy
Differential Revision: http://reviews.llvm.org/D7874

llvm-svn: 230523