Timeout error on download when /android/cache is missing

[Jani Monoses]

Firs off, the main bug here is /android/cache not being mounted on a device being brought up, so not s-i related.
However the error system-image-cli -n gives is a Timeout after 10 minutes. The download should proceed well, the network is working, so I think it may misinterpret the lack of files in /android/cache as a downloading error? I have not looked at the code hence the bold speculation :)

While the porters should make sure they have everything mounted correctly, s-i checking whether the destination exists would allow a quicker error message.

The end of the traceback:

[systemimage] Mar 05 14:45:36 2015 (4863) system-image-cli exception
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/systemimage/main.py", line 289, in main
    state.run_until('download_files')
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 175, in run_until
    step()
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 206, in _get_blacklist_1
    'image-master', 'gpg/image-master.tar.xz', 'archive-master')
  File "/usr/lib/python3/dist-packages/systemimage/keyring.py", line 107, in get_keyring
    (ascxz_src, ascxz_dst),
  File "/usr/lib/python3/dist-packages/systemimage/download.py", line 294, in get_files
    raise TimeoutError
TimeoutError