Check for enough signatures before sending a transaction to the node

[DOBEN]

Task description

When creating a transaction, and not signing it with enough signatures, and sending it off to the node, the node responds with the error:

Error: Transaction not accepted by the node: GRPC response with status 'INVALID_ARGUMENT': GRPC error: Verification failed.

This can happen when creating a transaction (signing it with some keys but not enough) and sending it off to the node OR
when loading a partially signed transaction (without enough signatures) from a JSON file and sending it off to the node.

Concordium-client should check before sending off a transaction to the node, if enough signatures are present and otherwise give enough information about which signatures are missing/required.