Add coverage calculation for error codes

[esabitova]

Errors can have different codes that should be covered by tests
For example for 409 we might have multiple error codes: TYPE_NOT_SUPPORTED, USERNAME_NOT_UNIQUE, EMAIL_NOT_UNIQUE and so on.
In json it looks like this:

It would be nice to add the ability to calculate coverage by error codes because there should be a separate test case for each error code.