Run 'UploadStashFile' hook for chunked uploads too

Run 'UploadStashFile' hook for chunked uploads too

In UploadFromChunks, doStashFile() only stashes the current chunk,
and stashing the whole file after all chunks are uploaded is done
in concatenateChunks().

Also more custom code on top of custom code to handle ApiMessage
errors, because action=upload is special.

Follow-up to 2ee27d9a4de163dc3a2b752823a04f5338005627.

Change-Id: I968280353f15bbca9b1059631fa2cfd7c3cd2f1d