unclear error "Collections across multiple backends"

[soxofaan]

openeo-aggregator/src/openeo_aggregator/backend.py

Line 326 in 4e5e79f

message=f"Collections across multiple backends ({union}): {collections}."

This error is unclear to user, e.g.

Collections across multiple backends ({'cdse', 'terrascope'}): {'SENTINEL1_GRD', 'ESA_WORLDCEREAL_TEMPORARYCROPS', 'SENTINEL2_L2A', 'COPERNICUS_30', 'ESA_WORLDCEREAL_WINTERCEREALS', 'ESA_WORLDCEREAL_MAIZE'}

suggests that all these collections are available on both cdse and terrascope

while it's actually the other way around: there is not a single backend that provides each of the collections, which does not allow naive (single backend) dispatching