Improve error handling

[mariavarvaroi]

Return an error in these scenarios:

• the response status is 200, but it contains an errors field
• the data field is missing
• the fields for the response that are supposed to be arrays have length 0

Test fixes:
Now that we return errors for length 0 arrays, we need to mock graphql responses for certain tests

[mksh]

It seems there are no tests right now that would check new error handling code; while I don't think we need to cover every API method error handling, if we extract common error handling logic it can be tested just once for 1 API method.

While it's not possible to make real Stakewise backend to return errors on-demand, we can use mocking of fetch method (inside connector.graphqlRequest) to emulate response containing errors, see examples via https://www.leighhalliday.com/mock-fetch-jest

[welldan97]

Looking good!

[mksh]

LGTM