Add Exception For xforms generation when Mode is missing in parameters

[JulienCarmona]

Feature Description

Add an exception that inform the user that his parameters.xml is missing <mode>

Use Case

When user use this endpoint https://api-generation-questionnaires.insee.fr/swagger-ui/index.html#/Generation%20of%20questionnaire%20(custom%20parameters)/generateXformsCustomParams
If there is no <mode> inside parameters.xml file, throw an exception

Proposed Solution

Return error code should be 400 BAD REQUEST

[nsenave]

J'allais estimer à 2 points mais on a eu des surprises l'autre fois, mais normalement tout est mis en place pour que ça soit facile à faire

=> partons sur 3 points