Fatal error: Call to undefined function error_handler() in cosign.module on line 200

When cosign is incorrectly configured (in my case, not enabled in the apache or .htaccess config) this error presents.

Seems that it checks for the $_SERVER['REMOTE_USER'] variable, which isn't set as cosign isn't configured correctly.

I can't find an error_handler function in any of the code base on my installation, so I've switched it to using drupal_error_handler which has an additional argument for context. The context argument seems to only be used for debugging database errors so I've passed zero (which I'm not sure is the right thing to do).

Please see the attached patch.