[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
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