The DMCA handling added in PR #454 had a bug: make_request_with_retry()
raises HTTPError before retrieve_data() could check the status code via
getcode(), making the case 451 handler dead code. This also affected
HTTP 403 TOS violations (e.g. jumoog/MagiskOnWSA).

Fix by catching HTTPError in retrieve_data() and converting 451 and
blocked 403 responses (identified by "block" key in response body) to
RepositoryUnavailableError. Non-block 403s (permissions, scopes) still
propagate as HTTPError. Also handle RepositoryUnavailableError in
retrieve_repositories() for the --repository case.

Rewrote tests to mock urlopen (not make_request_with_retry) to exercise
the real code path that was previously untested.

Closes #487