Fix/polling errors

[Jakub-Vacek]

Description

This PR partially fixes: https://github.com/superfaceai/cli/issues/324 It adds use of userError to function asserting correct response from server. This allows to print out correct error message and exit the process.

Motivation and Context

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] I have updated the documentation accordingly. For updating Oclif commands documentation use oclif-dev.
• [ ] I have read the CONTRIBUTION_GUIDE document.
• [ ] I haven't repeated the code. (DRY)
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.