Extend remote fallback for corrupted files

[Kruithne]

It is possible for local installations to be partial, meaning that certain files may be missing as they have not yet been streamed into the local data files, most likely due to not being required by the game yet. In the event of a missing file, wow.export will fallback to streaming this file from the remote server and then caching it locally.

It is possible for local installations to have corrupted files that cannot be parsed or do not match the checksum provided for said file. To cover this scenario, we should extend the remote fallback functionality used for missing files to also cover corrupted files.

An important note is that we should not patch the users local installation in the event of any missing or corrupted files. All fallback files must be stored within our own cache.