Some error messages should be considered as warning in signer registration to aggregator

[jpraynaud]

Why

Some error in the signer registration to the aggregator should be considered as warning and not impact the error metrics (e.g. registration round is not opened yet by the aggregator).

What

Handle the errors that should be considered as warnings in the signer registration to the aggregator.

How

• Introduce a new 550 error code in the aggregator when signer registration is not opened yet
• If the error is considered as a warning, the signer_registration_success_since_startup_counter and get_signer_registration_total_since_startup_counter metric should be incremented
• Create ADR to specify the reserved ranges of 4XX and 5XX codes that we can use in the Mithril nodes (see https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml)