[lldb-vscode] Report an error if an invalid program is specified.

Previously if an invalid program was specified, there was a bug
which, when we attempted to launch the program, would report that
the operation succeeded, causing LLDB to then hang while waiting
indefinitely to receive some events from the process.

After this patch, when an invalid program is specified, we immediately
return to vs code with an error message that indicates that the
program can not be found.

Differential Revision: https://reviews.llvm.org/D59114

llvm-svn: 355656