Server should avoid displaying full path on missing file error

[872342]

Background information

• Dendrite version or git SHA: Dendrite 0.13.1+a48c7d3
• SQLite3 or Postgres?: Postgres
• Running in Docker?: No
• go version: go1.20.6 linux/amd64
• Client used (if applicable): N/A

Description

• What is the problem: Deleting a file or directory from media_store directory results in system path name error
• Who is affected: Anyone missing files in their media_store dir.
• How is this bug manifesting: In the HTTP response 404 errcode error
• When did this first appear: Unknown

Steps to reproduce

• upload a file to homeserver
• delete file/directory in media_store directory
• try to download the file

When trying to request a file which doesn't exist
The response error is a generic "File not found"
{"errcode":"M_NOT_FOUND","error":"File not found"}

But if the file or parent directory no longer exist in the media_store directory, the error includes the full path
{"errcode":"M_NOT_FOUND","error":"Failed to download: os.Open: open /home/user/dendrite/media_store/path/to/file: no such file or directory"}

The response returns to the "File not found" error when the file is removed from the mediaapi_media_repository table and cache is cleared.
But a more generic error message would be better to avoid displaying system info publicly.