Colossus: explicitly handle nonexistent asset case

[kdembler]

Problem

If I try to upload asset (identified by (bagId, dataObjectId) pair) that doesn't exist (at least one of the IDs is wrong), Colossus will actually not let me know. Instead, as seen below, the upload will fail at asset size verification, telling me that there's size mismatch, when the nature of error is actually quite different.

https://github.com/Joystream/joystream/blob/97b311f071f6f76b5de78da281d88188466c63ce/storage-node/src/services/webApi/controllers/filesApi.ts#L272-L279

Proposal

We should explicitly handle the case where dataObject wasn't found and provide meaningful error message to help with debugging.

┆Issue is synchronized with this Asana task by Unito

[bedeho]

Adding to mainnet scope, great find :)

[bedeho]

Done.