Document download endpoint should return 404 if physical file does not exist

[plittlewood-rpt]

Describe the bug
Logged from Zendesk case 912

The document download endpoint (GET platform.reapit.cloud/documents/{id}/download) currently throws an exception and returns a 500 error if the physical file does not exist in the customers document storage location. This should really be a 404. We recently introduced a HEAD method which behaves in the correct manner, so this should be easily resolved

To Reproduce
Steps to reproduce the behaviour eg:

1. Upload a document then delete the physical file asset so the database record becomes orphaned
2. Try to download the document via the API. Observe 500 error

Specification

• Project: Reapit.Services.Documents
• Fix bug as above, 404 should be returned when physical asset does not exist