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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[github-actions]

We want to keep our issues up to date and active. This issue hasn't seen any activity in the last 75 days.
We have applied the _flag:stale label to indicate that this issue should be reviewed again.
When you review, please reread the spec and then apply one of these three options:

• prioritize: apply the flag:schedule label to suggest moving this issue into the backlog now
• close: if the issue is no longer relevant, explain why (give others a chance to reply) and then close.
• archive: sometimes an issue has important information or ideas but we won't work on it soon. In this case
  apply the someday label to show that this won't be prioritized. The stalebot will ignore issues with this
  label in the future. Use sparingly!