We try to do a JSON response if Python is not present, but it assumes sed. We should cleanly
exit on errors.
Before:
```
$ PATH=/tmp ./plugins/wss-proxy/wss-proxy
Something
./plugins/wss-proxy/wss-proxy: 12: sed: not found
{"jsonrpc":"2.0","id":,"result":{"disable":"No python3 binary found"}}
```
After:
```
$ PATH=/tmp ./plugins/wss-proxy/wss-proxy
something
./plugins/wss-proxy/wss-proxy: 12: sed: not found
```
Reported-by: Christian Decker
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
2c57347033