fix: surface backend error message on failed video upload

[santhosh-apphelix-2u]

Summary

• Studio's video upload page always displayed a generic "Failed to add {filename}." message whenever the create-upload-url request failed, regardless of the actual reason returned by the backend.
• This made real failures (e.g. non-ASCII characters in the filename) impossible to diagnose from the UI, and led to support/engineering having to dig through server logs to find the actual cause.
• This change surfaces the backend's actual error message when present, falling back to the previous generic message only if the backend didn't return one.

Root cause context (LP-1036)

Partner-reported "Failed to add video" errors were traced to filenames containing non-ASCII characters (e.g. an en dash – instead of a hyphen -), which videos_post rejects with a specific error message that the frontend was discarding. This restriction is intentional — the filename is sent to S3 as object metadata, which must be ASCII — so the backend validation itself is not being changed here, only how its result is communicated to the user.

Changes

• src/files-and-videos/videos-page/data/thunks.js: pass the backend's error response field through to the failAddVideo action.
• src/files-and-videos/videos-page/data/slice.js: use that message when building the displayed error, falling back to the generic text if none was provided.