maas-import-pxe-files fails when md5 checksums can't be downloaded

The maas-import-pxe-files script fails in fetch_server_md5sums when the MD5SUMS file for an architecture and release is unavailable. It should skip that image and move on.

Example of the failure: http://paste.ubuntu.com/6174817/

With this fixed, the script itself should work without an explicit DISTRO_SERIES setting — which is currently breaking. It can just attempt to download all supported releases, and succeed even though Lucid (10.04) is still supported but has no images available. The ephemerals script will still try and fail, but we're fixing that with the Python/simplestreams rewrite.