Views validation: guard against non-existent flags

(Of course, it's for D6 as well.)

This validate() method doesn't run for existing views. We may also want to make sure such views fail gracefully (one user reports getting a WSOD in this case).

Nate, are you fine with this patch?

This patch is "safe". It doesn't alter existing code. It just adds validation.

This validate() method doesn't run for existing views.

(I wasn't clear here: I meant to say that validate() isn't called when a view is merely run. It's called when one saves a view or previews it.)

I'm marking this a duplicate of #969102: Don't specify a default flag.

Re-opening this issue.

#994674: Flag does not exist was marked a dup of this issue.

I'm going to commit the following patch.

What was added?
- Relationships now have a get_flag() method that does validation at "run time".
- query() methods now exit if get_flag() returns NULL. get_flag() prints a message in case of missing flag.

Committed.
http://drupal.org/cvs?commit=468574
http://drupal.org/cvs?commit=468578

(Marking this 'critical', as not having this guard results in WSODs.)