improve error handling of read_vector on geojson URL

[soxofaan]

from https://discuss.eodc.eu/t/fit-class-random-forest-413-request-too-large/396:

process graph uses read_vector with a geojson URL like https://eolab.eodc.eu/hub/user-redirect/lab/tree/blabla/features.json, which forwards to a HTML login page (https://eolab.eodc.eu/hub/login?next=%2Fhub%2Fuser-redirect%2Flab%2Ftree%2Fblabla%2Ffeatures.json), which causes the json parsing to fail with

OpenEoApiError: [500] Internal: Server error: JSONDecodeError('Expecting value: line 4 column 1 (char 3)')

Confusingly, the HTML had three empty lines, causing the error to mention "line 4", which gave the impression that the URL at least started with valid JSON, so it took some time to realize that the geojson response wasn't valid JSON at all.

Some improvements to help with debugging this problem: add some logging around read_vector of what URL is loaded, what the content type is, warn if it is not json ( but still allow it: github raw urls have content type "text/plain"), improve the information of the exception (e.g. include the url)

[soxofaan]

Another feature idea: users might use a github url like https://github.com/Open-EO/openeo-python-driver/blob/master/tests/data/geojson/FeatureCollection01.json (which does not work because there is a whole HTML page around the actual json). maybe we can easily convert that to a usable raw URL (https://raw.githubusercontent.com/Open-EO/openeo-python-driver/master/tests/data/geojson/FeatureCollection01.json in this case)

[soxofaan]

closing this for now, the current change regarding logging and error message should already be enough of an improvement