a few 500 Internal Server Errors (SQLITE_CONSTRAINT_PRIMARYKEY)

[pohutukawa]

Describe the bug
Exploring the wallet-api, I have stumbled upon a few unhandled exceptions leading to 500 Internal Server Errors. In this case they were all tied to primary key constraint errors in the DB, so I would guess that they're likely applicable to other persistence back-ends as well.

Specifically they have occurred in the following cases:

• Create an account if the account ID (email) already exists. ({"exception":true,"status":"Internal Server Error","code":"500","message":"Could not register user: Account already exists!"})
• Create a DID (Web) if the DID already exists. ({"exception": true, "status": "Internal Server Error", "code": "500", "message": "org.sqlite.SQLiteException: [SQLITE_CONSTRAINT_PRIMARYKEY] A PRIMARY KEY constraint failed (UNIQUE constraint failed: wallet_dids.wallet, wallet_dids.did)"})
• Import a key (JWK) if the key ID (auto-generated) already exists. ({"exception":true,"status":"Internal Server Error","code":"500","message":"org.sqlite.SQLiteException: [SQLITE_CONSTRAINT_PRIMARYKEY] A PRIMARY KEY constraint failed (UNIQUE constraint failed: wallet_keys.wallet, wallet_keys.kid)"})

To Reproduce
Steps to reproduce the behaviour:

1. Import a key in JWK format: POST /wallet-api/wallet/{wallet}/keys/import
2. Import that same key in JWK format again.
3. Observe the API returning a 500 error due to an unhandled exception.

Expected behaviour
A 409 Conflict or 400 Bad Request error response after successfully handling the exception.

Actual behaviour
An unhandled exception/not gracefully handled exception leading to a 500 Internal Server Error response without significant meaningful context assisting the user to resolve the situation.
eenshots to help explain your problem.

Environment
Desktop (please complete the following information):

• Browser: curl command line or other scripted (e.g. via Python requests)
  Additional context
  Add any other context about the problem here.