Handle bad layers at QGIS project loading and throw exception

[rldhont]

qgis/QGIS#9991

[dmarteau]

Bad layers are dismissed at QgsProject.read(...), so we end without the bad layers. it seems that there is no flags or status that enable to check if layer were dismissed, no exception is thrown.....

[dmarteau]

Fixed in 5e8ad90

Need to implement a QgsProjectBadLayerHandler to catch up invalid layers. This behavior can be activated/deactivated with the cache.strict_check (QGSRV_CACHE_STRICT_CHECK) option.

[dmarteau]

Note that the server will return a 422 HTTP error (Unprocessable entity) and not a 500.