download errors of encrypted attachments are reported as crypto failures - "Error decrypting image"

[richvdh]

If you get an error when downloading an attachment (eg because you can't talk to your HS, or because it's on a remote HS which your HS can't access), we try to decrypt the error message, and hence show a decryption error.

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⚠ Error decrypting image

[richvdh]

related to #2749

[dkasak]

This also happens if your Synapse has the max_upload_size setting set to a lower value than the file size of the attachment it's trying to fetch from a remote server.