Alexa: Fix execute on multiple device feature + reject when device is not found

[Pierre-Gilles]

Pull Request check-list

To ensure your Pull Request can be accepted as fast as possible, make sure to review and check all of these items:

• If your changes affects code, did your write the tests?
• Are tests passing? (npm test on both front/server)
• Is the linter passing? (npm run eslint on both front/server)
• Did you run prettier? (npm run prettier on both front/server)
• If you are adding a new features/services, did you run integration comparator? (npm run compare-translations on front)
• Did you test this pull request in real life? With real devices? If this development is a big feature or a new service, we recommend that you provide a Docker image to the community for testing before merging.
• If your changes modify the API (REST or Node.js), did you modify the API documentation? (Documentation is based on comments in code)
• If you are adding a new features/services which needs explanation, did you modify the user documentation? See the GitHub repo and the website.
• Did you add fake requests data for the demo mode (front/src/config/demo.js) so that the demo website is working without a backend? (if needed) See https://demo.gladysassistant.com.

NOTE: these things are not required to open a PR and can be done afterwards / while the PR is open.

Description of change

Please provide a description of the change here. It's always best with screenshots, so don't hesitate to add some!

[relativeci]

Job #312: Bundle Size — 6.74MB (0%).

50b42a1 vs 0525ac2

No changes

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View Job #312 report on app.relative-ci.com

[codecov]

Codecov Report

Merging #1536 (3d7c7fe) into master (0525ac2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1536   +/-   ##
=======================================
  Coverage   96.59%   96.59%           
=======================================
  Files         627      627           
  Lines        9155     9158    +3     
=======================================
+ Hits         8843     8846    +3     
  Misses        312      312           

Impacted Files	Coverage Δ
server/services/alexa/lib/alexa.onExecute.js	100.00% <100.00%> (ø)
server/services/alexa/lib/alexa.onReportState.js	100.00% <100.00%> (ø)

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