Check for write permisions on data directory

[marcovanbeek]

In server.php there is a line that says:
$lockBackend = new DAV\Locks\Backend\File('data/locks.dat');
The function this calls does not check if the file/directory is writeable, and with Windows10 clients this produces the "file too big" error.

Checking that the data directory is writeable at this point would help this issue to be better logged.