Media / Async: Network failures during iCloud fetch aren't recoverable

I'm not entirely sure if this is fixable or not...

Expected behavior

If my post fails to upload, I expect to be able to tap Retry and for it to upload again successfully.

Actual behavior

If there is a network failure or other error when fetching media from a user's iCloud library, attempts to retry the upload always fail.

Steps to reproduce the behavior

• Start a new post, and add media that is in the user's iCloud Photos library, but not on device.
• Before the media finishes processing, publish async and then turn on airplane mode.
• The post will fail for the following reason:

2018-04-05 09:47:03:409 WordPress[5788:814178] Error occurred importing to Media with system error, code: 81, error: Error Domain=CloudPhotoLibraryErrorDomain Code=81 "Failed to download CPLResourceTypeOriginal" UserInfo={NSLocalizedDescription=Failed to download CPLResourceTypeOriginal, NSUnderlyingError=0x1c265d100 {Error Domain=CKErrorDomain Code=3 "(null)" UserInfo={NSUnderlyingError=0x1c2840510 {Error Domain=NSURLErrorDomain Code=-1009 "The Internet connection appears to be offline." UserInfo={NSLocalizedDescription=The Internet connection appears to be offline.}}}}}

• If you switch off airplane mode and attempt to retry the upload of the failed post, it will always fail for the following reason:

2018-04-05 10:19:16:891 WordPress[5954:830625] Error uploading media: Error Domain=MediaServiceErrorDomain Code=0 "Media doesn't have an associated file to upload." UserInfo={NSLocalizedDescription=Media doesn't have an associated file to upload.}

Tested on iPad Pro 9.7", iOS 11.3, WPiOS develop (9.7 / 9.8)

cc @SergioEstevao

Start by reproducing the async publish flow with media stored only in the user's iCloud Photos library, then compare the initial failure with a retry after connectivity returns. Trace the iCloud media fetch and failed-post retry paths described in the report. Done means a network failure during fetching does not leave the media without an associated file, and Retry can upload the post successfully.