check if the decrypted private key is valid on login and on read/write files

As discussed by @blizzz on #3555:
>What will actually happen when such a users continues to access ownCloud via webdav? will encrypted files be synced down? or is 4xx thrown?

With this PR the following limitations occur if the user password was changed from the outside.
- Login on the Web will not work
- Download and upload files via WebDAV will also not work

if this happen we log this errors in our owncloud.log so an admin can handle this.

This is not the best solution but it prevents the user running into problems where files are messed up. 

cc @blizzz @karlitschek @schiesbn