Update marketplace to single output

[NicolasMahe]

The system service marketplace need to be updated to single output.

• update mesg.yml. Already done
• Update codebase to use new mesg-js beta
  • return only outputData in task function
  • throw error if any in task function

[NicolasMahe]

This PR is working and ready to review.
The last todo can be done in another PR after the mesg-js lib has been updated!

[antho1404]

➜  marketplace git:(feature/single-output-marketplace) docker build .
Sending build context to Docker daemon  400.9kB
Step 1/7 : FROM node:10.11.0
 ---> 8672b25e842c
Step 2/7 : WORKDIR /app
 ---> Using cache
 ---> 7516b530e8ed
Step 3/7 : COPY ./package* ./
 ---> 1b00b8edcef5
Step 4/7 : RUN npm install
 ---> Running in 63bd2a87dba3
Unhandled rejection Error: Command failed: /usr/bin/git checkout beta
error: pathspec 'beta' did not match any file(s) known to git.

    at ChildProcess.exithandler (child_process.js:289:12)
    at ChildProcess.emit (events.js:182:13)
    at maybeClose (internal/child_process.js:962:16)
    at Socket.stream.socket.on (internal/child_process.js:381:11)
    at Socket.emit (events.js:182:13)
    at Pipe._handle.close (net.js:606:12)

[antho1404]

lib fixed, ready to merge for me

[NicolasMahe]

@antho1404 modif are good 👍