Check if the cached js file exists
Check if the cached js file exists

Fixes #8705

If the file does not exist (for whatever reason). It is never cached. No
matter what the depscache etc tell you.

Signed-off-by: Roeland Jago Douma <982d370f7dc34a05b4abe8788f899578d515262d@famdouma.nl>

nextcloud/server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server