ViewManager needs to wrap http calls for proper exceptions

For instance, drop_design_document just makes a call to the core bucket manager, and it will raise HttpExceptions when there are failures. Lets (for the short term) make a wrapper class for this. Later, a better approach is of course to rip out the v2 stuff entirely, and just make these http calls directly (reusing the core code as much as possible).

This makes one of the viewmgt_t tests fail (against a real server, skipped in the mock).