remove (unnecessary) state field
ignore 404 on historic root catalogs
documentation update
print error message for failed traversal
code cosmetics
silence error mesages in CatalogTraversal<>
TEST: CheckVisitedCatalogs() can ignore catalog count
TEST: make RevisionMap accessible by test cases
TEST: GetCatalog() to access random catalogs
TEST: MockObjectFetcher can hide catalogs
TEST: CatalogTraversal<> can ignore 404 on root catalogs
error handling in GarbageCollector<>
TEST: garbage with unreachable catalogs
TEST: check return value of Traverse()
TEST: check return value of Collect()
FIX: don't unnecessarily sweep