fix warning from scipy backend guess_can_open on directory

[germasch]

When passing a directory to open_dataset(), the scipy backend fails and a "RuntimeWarning: 'scipy' fails while guessing" is produced.

This affects me since I'm implementing a backend to read data written by the adios2 package, whose data "files" are actually a directory.

This tiny patch treats this case just like file not found, that is, the scipy backend will now return that it cannot open such a "file", but without raising an exception.

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[headtr1ck]

Looks like a useful addition, thanks!
Should we maybe catch a PermissionError as well?

And feel free to add an entry in whats-new.

[germasch]

I figured this change is too minor to justify a whats-new entry, but I'd be happy to do so if you'd like.

Good point about considering PermissionError, too. I just looked, and PermissionError is already special cased in the case of built-in backends, in that it's re-raised. I think that's the right thing to do -- a PermissionError is very likely not happening because it's a mismatched backend, but rather because of an actual permission problem, and the user will benefit from the specific failure (exception), as opposed to a generic "no matching backend found" error. So I think the answer is probably to propagate a PermissionError from plugin backends to the user as well. If you agree, I can make that change, and make sure that it behaves as desired.