Azure: raise FileNotFoundError when requested blob is missing from storage

[alxbridge]

This PR implements behaviour in AzureStorage._open similar to that already found in S3Storage._open, raising a FileNotFoundError if the blob doesn't exist in the storage.

[jschneier]

Thanks, makes sense to fix this.

Any idea about doing the same kind of RAII-style handling in init of AzureStorageFile rather than calling .exists()? Might be able to save an API call that way (which costs money).

[jschneier]

Can you change this to use open() as that is the public API. Also to use a with context handler.

[alxbridge]

I've just pushed commits to make these changes

[alxbridge]

Thanks, makes sense to fix this.

Any idea about doing the same kind of RAII-style handling in init of AzureStorageFile rather than calling .exists()? Might be able to save an API call that way (which costs money).

I'm not sure I understand the existing code / Azure SDK well enough to say, tbh. It looks like where that's done in the S3File class, it's conditional on the file being opened in read mode, but the init for AzureStorageFile isn't currently passed similar information on the intended mode for the file.