No need to test for arch validation

We no longer need to validate the architecture as the download
is no longer arch-specific, so fix the tests which check that
arch validation is working correctly.
We no longer need ARCHS, as we don't validate

The VersionsFetcher no longer fetches a download list for
a specified architecture, so we don't need an ARCHS constant
which enumerates the valid architectures.
[ArchiveFetcher] Add a catch-all for errors

In the download/unpack chain, we might get errors occurring at
unpack time. Add a handler to the done() call which catches
any errors coming from the Unpacker instance.