TDMQ REST API should return an error in case of entity category missing

[svituz]

Description

Using the REST API, when a client tries to make a POST of a new Source with an entity_type/entity_category pair which is not present in the DB, the server returns an error page
which states that a psycopg2 error occurred. The API should, instead, handle the error and return an appropriate response specifying it.

How to reproduce

In your local machine, launch the docker development env then, using your favorite client (e.g., postman), try to perform a POST to /api/v0.0/sources call using the following json body:

[{
	"id": "AN.EDGE.ID", 
	"alias": "AN.EDGE.ALIAS", 
	"entity_type": "NONEXISTENT-TYPE", 
	"entity_category": "NONEXISTENT-CAT", 
	"default_footprint": {
		"type": "Point", 
		"coordinates": ["0","0"]		
	}, 
	"stationary": true,
	"controlledProperties": [
		"humidity", 
		"temperature", 
		"dewpoint"
	], 
	"shape": [], 
	"description": {}
}]

Notice that the important attributes are entity type and category, other values are irrelevant.

What happens

The server returns a 500 Internal Server Error with a the exception:

"psycopg2.errors.ForeignKeyViolation: insert or update on table "source" violates foreign key constraint "source_entity_category_fkey" DETAIL: Key (entity_category, entity_type)=(Station, DeviceStatus) is not present in table "entity_type".

What should happen

The server should return an appropriate error code (TBD) and a JSON response with some error details.