api: Improve handling of privkey decryption errors

[kevinbader]

Closes #174.

Test:

1. Start blockchain
2. Start API with ORGANIZATION=A and ORGANIZATION_VAULT_SECRET=a
3. Run provisioning
4. Restart API with ORGANIZATION=B and ORGANIZATION_VAULT_SECRET=b
5. Run frontend
6. Log in with mstein and test

On the UI you should see "Incorrect password" and the logs should show this:

msg: "authentication failed: failed to decrypt the user's private key with the given organization secret (does the user mstein belong to organization ACMECorp2?): decryption failed"
error: {
  ...
}

[Stezido]

Test works as expected

[Stezido]

@kevinbader the messages sent to the frontend are a bit long to show them in the error snackbars. So we need a mapping on frontend side. If the frontend has only messages as information about the error occurred, how can it decide between for example 400 Bad Request:

• invalid user,
• invalid password,
• user exists in other organization

This issue is already described in #131

code is not returned due to wrong response schema