[lldb-vscode] Don't try to launch an invalid program.

If an invalid program is specified, lldb-vscode will send back a
response with "success" = false, but then will continue executing the
rest of request_launch(), try to launch the program anyway and try to
send another response (possibly using the `response` object after it was
moved).

This change adds a return statement so we stop executing the handler
after producing the first failing response.

Differential Revision: https://reviews.llvm.org/D59340

llvm-svn: 356110