Test and handle API errors

[maxkoryukov]

Currently, handling of API-errors is not thoroughly tested.

We can't provide week code, so, we must cover the code with tests, and ensure that the all errors are handled and provide clear descriptions and codes

Checklist:

• cover with tests the code-branch with error-handling of requests
• provide description for errors
• provide code for errors (when possible) - this will help to handle errors automatically [for end users!]