Fixed quota wrapper to not wrap failed fopen streams

When calling fopen() on some storage types, these return false instead
of throwing an exception.

This fix makes sure that in case the stream wasn't opened (for example
when a file doesn't exist any more) the stream isn't wrapped.

Also added 'rb' as another case that doesn't need to be wrapped.

Fixes #6832
Merge pull request #6838 from owncloud/core-quotawrapperfailedfopenfix

Fixed quota wrapper to not wrap failed fopen streams