Turn 451 and 410 of the name and id lookups into Fbe::Error

[Thayorns]

repo_id_by_name, repo_name_by_id and user_name_by_id turned only a 404 and a 403 into Fbe::Error. GitHub also answers 451 for a repository under a DMCA takedown and 410 for some removed resources, and these came out as raw Octokit::UnavailableForLegalReasons and Octokit::Deprecated, while the callers expect Fbe::Error, as #1227 shows.

Now the three lookups also rescue these two errors and raise the same "is not accessible" Fbe::Error. I kept the list explicit instead of rescuing every Octokit::ClientError, so a rate limit error still reaches the caller as it is.

Six tests stub each lookup with a 451 and with a 410 and expect Fbe::Error.

Closes #1227

[Thayorns]

@yegor256 take a look please, happy to clarify anything about this change.