[Fix] Handle non-JSON errors

[mgyucht]

Changes

Some errors returned by the platform are not serialized using JSON (see #998 for an example). They are instead serialized in the form "<ERROR_CODE>: ". Today, the SDK cannot parse these error messages well, resulting in a poor user experience.

This PR adds support for parsing these error messages from the platform to the SDK. This should reduce bug reports for the SDK with respect to unexpected response parsing. This PR also refactors the error deserialization logic somewhat to make it more extensible in the future for other potential error formats that are not currently handled.

Breaking Changes

This PR renames MakeUnexpectedError() to MakeUnexpectedResponse() in the apierr package. It also changes the return type to string. This makes the message easier to incorporate into error responses that only depend on the string representation of the error, as well as allows us to start the message with a capital letter, as it is a complete sentence.

The error message for failed deserialization of valid responses has changed slightly, from unexpected error handling request to failed to unmarshal response body. The rest of the message is identical.

Tests

Refactored unit tests to a table-driven test case, and added four new cases: one for error details (not previously covered), one for the regular happy path, one for unexpected responses, and one for the new error message format.

• make test passing
• make fmt applied
• relevant integration tests applied

[renaudhartert-db]

No concern about the logic, left a few nit comments.

[renaudhartert-db]

Thanks!