adt-buildvm-ubuntu-cloud crashed with urllib.error.HTTPError in http_error_default(): HTTP Error 404: Not Found

Supplying an arch that is not supported results in python trace. This is ugly and should be trapped and provide a more user friendly error message.

$ adt-buildvm-ubuntu-cloud -a aarch64 -r yakkety
WARNING: python-distro-info not installed, falling back to determining default release from currently installed release
Downloading https://cloud-images.ubuntu.com/yakkety/current/yakkety-server-cloudimg-aarch64.img...
Traceback (most recent call last):
  File "/usr/bin/adt-buildvm-ubuntu-cloud", line 349, in <module>
    image = download_image(args.cloud_image_url, args.release, args.arch)
  File "/usr/bin/adt-buildvm-ubuntu-cloud", line 185, in download_image
    headers = urllib.request.urlretrieve(image_url, local_image, report)[1]
  File "/usr/lib/python3.5/urllib/request.py", line 187, in urlretrieve
    with contextlib.closing(urlopen(url, data)) as fp:
  File "/usr/lib/python3.5/urllib/request.py", line 162, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib/python3.5/urllib/request.py", line 471, in open
    response = meth(req, response)
  File "/usr/lib/python3.5/urllib/request.py", line 581, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.5/urllib/request.py", line 509, in error
    return self._call_chain(*args)
  File "/usr/lib/python3.5/urllib/request.py", line 443, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.5/urllib/request.py", line 589, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 404: Not Found

ProblemType: Crash
DistroRelease: Ubuntu 16.10
Package: autopkgtest 3.20.7
ProcVersionSignature: Ubuntu 4.4.0-23.41-generic 4.4.10
Uname: Linux 4.4.0-23-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu4
Architecture: amd64
CrashCounter: 1
CurrentDesktop: Unity
Date: Fri May 27 10:26:55 2016
EcryptfsInUse: Yes
ExecutablePath: /usr/bin/adt-buildvm-ubuntu-cloud
InstallationDate: Installed on 2016-04-29 (27 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
InterpreterPath: /usr/bin/python3.5
PackageArchitecture: all
ProcCmdline: /usr/bin/python3 /usr/bin/adt-buildvm-ubuntu-cloud -a aarch64 -r yakkety
PythonArgs: ['/usr/bin/adt-buildvm-ubuntu-cloud', '-a', 'aarch64', '-r', 'yakkety']
SourcePackage: autopkgtest
Title: adt-buildvm-ubuntu-cloud crashed with urllib.error.HTTPError in http_error_default(): HTTP Error 404: Not Found
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip kvm libvirt libvirtd lpadmin plugdev sambashare sudo