Use Axios validateStatus option to handle expectable API errors

[arildm]

Mink-backend responds with error statuses (4xx, 5xx) in some cases that in the frontend are not seen as errors, for example:

• /run-sparv and friends return 500 if the job cannot be started due to invalid data/config
• /load-config returns 404 when corpus exists but there's no config file

Axios has a validateStatus option (see Handling Errors) that can be used to declaratively filter which status should be considered errors.