Refactor warning raising

[alyssadai]

Warnings related to local nodes not being available could be handled by parse_nodes_as_dict().

We may also want to make the warning less generic / confusing (are there no local nodes, did the file not exist, was the file empty ...)

federation-api/app/api/utility.py

Lines 54 to 58 in 308e66e

	if not local_nodes:
	warnings.warn(
	f"No local Neurobagel nodes defined or found. Federation will be limited to nodes available from the Neurobagel public node directory {node_directory_url}. "
	"(To specify one or more local nodes to federate over, define them in a 'local_nb_nodes.json' file in the current directory and relaunch the API.)\n"
	)

This would also require updating several of the tests.

Originally posted by @surchs in #39 (review)

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