[FIX JENKINS-35845] Internationalisation for Blue Ocean and JDL (#2586)

* Load i18n resource bundles from plugins if not found in jenkins core

Signed-off-by: Thorsten Scherler <scherler@gmail.com>

* Issue 404 response for missing i18n resource bundles

Currently issues a 200 with an "error" response payload. This change still issues the error response payload, but also sets the HTTP response.

Signed-off-by: Thorsten Scherler <scherler@gmail.com>

* [JENKINS-35845] Fix test since we return now a 404

* [JENKINS-35845] add test for getting locale from plugin. fix comments from oleg.

* [JENKINS-35845] Fix description

* [JENKINS-35845] Update PR with comments from Oleg

* [JENKINS-35845] Add feedback from tom

* eslint - formating changes and fix offences

* eslint - formating changes and fix offences

* [JENKINS-35845] remove code concerning 404 response. Fix resourceBundle test by prevent NPE to happen

* [JENKINS-35845] Link to issue on which we introduced the test