bug in sourmash.load_file_as_signatures(...) when loading non-existent .zip file

[ctb]

This:

sourmash.load_file_as_signatures('foo87878.zip')

yields a confusing error about manifests, when foo87878.zip doesn't exist. I think it should instead say "this file doesn't exist" 😄

[ctb]

Even easier,

sourmash sig describe foo87878.zip

produces a long traceback, ending with:

...
  File "/Users/t/dev/sourmash/src/sourmash/index.py", line 526, in __init__
    self._load_manifest()
  File "/Users/t/dev/sourmash/src/sourmash/index.py", line 538, in _load_manifest
    manifest_data = self.storage.load('SOURMASH-MANIFEST.csv')
  File "/Users/t/dev/sourmash/src/sourmash/sbt_storage.py", line 210, in load
    raise FileNotFoundError(path)
FileNotFoundError: SOURMASH-MANIFEST.csv