Bug: using -o with bad .zip file gives a terrible error messagae

[ctb]

discover by @dkoslicki - the following commands work:

rm test.zip
sourmash sig cat tests/test-data/prot/protein.zip -o test.zip

but then this fails b/c test.zip is a badly formatted zip file:

rm test.zip; touch test.zip
sourmash sig cat tests/test-data/prot/protein.zip -o test.zip

and you get the error message:

== This is sourmash version 4.4.4.dev4+gceb590ce. ==
== Please cite Brown and Irber (2016), doi:10.21105/joss.00027. ==

Traceback (most recent call last):
  File "/Users/t/miniforge3/envs/py38/bin/sourmash", line 33, in <module>
    sys.exit(load_entry_point('sourmash', 'console_scripts', 'sourmash')())
  File "/Users/t/dev/sourmash/src/sourmash/__main__.py", line 13, in main
    return mainmethod(args)
  File "/Users/t/dev/sourmash/src/sourmash/cli/sig/cat.py", line 54, in main
    return sourmash.sig.__main__.cat(args)
  File "/Users/t/dev/sourmash/src/sourmash/sig/__main__.py", line 95, in cat
    save_sigs.open()
  File "/Users/t/dev/sourmash/src/sourmash/sourmash_args.py", line 1115, in open
    storage = ZipStorage(self.location, mode="w")
  File "/Users/t/dev/sourmash/src/sourmash/sbt_storage.py", line 103, in __init__
    self.__inner = _RwZipStorage(path)
  File "/Users/t/dev/sourmash/src/sourmash/sbt_storage.py", line 222, in __init__
    self.zipfile = zipfile.ZipFile(path, 'r')
  File "/Users/t/miniforge3/envs/py38/lib/python3.8/zipfile.py", line 1269, in __init__
    self._RealGetContents()
  File "/Users/t/miniforge3/envs/py38/lib/python3.8/zipfile.py", line 1336, in _RealGetContents
    raise BadZipFile("File is not a zip file")
zipfile.BadZipFile: File is not a zip file
Exception ignored in: <function _RwZipStorage.__del__ at 0x12d4fa0d0>
Traceback (most recent call last):
  File "/Users/t/dev/sourmash/src/sourmash/sbt_storage.py", line 400, in __del__
    self.close()
  File "/Users/t/dev/sourmash/src/sourmash/sbt_storage.py", line 326, in close
    if self.zipfile is not None or self.bufferzip is not None:
AttributeError: '_RwZipStorage' object has no attribute 'zipfile'

which is really not very helpful 😆 .