writing to locked files/folders give only a generic error message

[individual-it]

Steps to reproduce

1. lock a folder e.g. curl -u admin:admin http://localhost/owncloud-core/remote.php/webdav/folder1 -X LOCK -d '<?xml version="1.0" encoding="UTF-8"?><d:lockinfo xmlns:d="DAV:"><d:lockscope><d:shared/></d:lockscope> </d:lockinfo>'
2. try to upload a file into that locked folder via web

Expected behaviour

Error message should say Upload failed because the current folder is locked.

Actual behaviour

Message only says: File upload failed…

See also discussion in #5728 (review)