art install raises a KeyError exception if an artifact isn't available in the cache

[haboustak]

If the art install command is run without art download or if an archive file is removed from the cache directory, the command raises a KeyError exception and prints a traceback.

$ python3 -m art install
Traceback (most recent call last):
FileNotFoundError: [Errno 2] No such file or directory: '/home/user/.cache/art/project/499184.zip'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):

  File "kosma/gitlab-art/art/_cache.py", line 35, in get
    raise KeyError(filename)