Empty endpoints raise errors on validation

[ml-evs]

As an example optimade-validator https://aiida-dev.materialscloud.org/tc-applicability/optimade -vv ends up reporting two failures:

FAILURES
========

https://aiida-dev.materialscloud.org/tc-applicability/optimade - _recurse_through_endpoint - failed with error
	ResponseError: Unable to generate filters for endpoint structures: no valid entries found.
	Traceback (most recent call last):
	  File "/home/mevans/src/optimade/optimade-python-tools/optimade/validator/utils.py", line 300, in wrapper
	    raise exc
	  File "/home/mevans/src/optimade/optimade-python-tools/optimade/validator/utils.py", line 290, in wrapper
	    result, msg = test_fn(validator, *args, **kwargs)
	  File "/home/mevans/src/optimade/optimade-python-tools/optimade/validator/validator.py", line 350, in _recurse_through_endpoint
	    f"Unable to generate filters for endpoint {endp}: no valid entries found."
	optimade.validator.utils.ResponseError: Unable to generate filters for endpoint structures: no valid entries found.
	
https://aiida-dev.materialscloud.org/tc-applicability/optimade/structures?page_limit=5&response_fields=chemical_formula_descriptive,chemical_formula_hill,elements_ratios,species,chemical_formula_reduced,cartesian_site_positions,assemblies,nelements,chemical_formula_anonymous,immutable_id,structure_features,elements,nsites,species_at_sites,lattice_vectors,dimension_types,nperiodic_dimensions,last_modified - _get_single_id_from_multi_entry_endpoint - failed with error
	ResponseError: No entries found under endpoint to scrape ID from. This may be caused by previous errors, if e.g. the endpoint failed deserialization.
	Traceback (most recent call last):
	  File "/home/mevans/src/optimade/optimade-python-tools/optimade/validator/utils.py", line 300, in wrapper
	    raise exc
	  File "/home/mevans/src/optimade/optimade-python-tools/optimade/validator/utils.py", line 290, in wrapper
	    result, msg = test_fn(validator, *args, **kwargs)
	  File "/home/mevans/src/optimade/optimade-python-tools/optimade/validator/validator.py", line 1087, in _get_single_id_from_multi_entry_endpoint
	    "No entries found under endpoint to scrape ID from. "
	optimade.validator.utils.ResponseError: No entries found under endpoint to scrape ID from. This may be caused by previous errors, if e.g. the endpoint failed deserialization.

These should be replaced by warnings.