Handle lack of basic auth better

[soxofaan]

from https://github.com/openEOPlatform/architecture-docs/issues/107

python client assumes basic auth is always available
when not, it should handle report that in a clean way

now it raises:

OpenEoApiError: [404] NotFound: 404 Not Found: The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.