Werk #12132: Fix view crash when linking to views with missing context

Previous 2.0 versions produced crash reports (KeyErrors) when a column was linking to another view while a mandatory context information was missing in the source row. This could e.g. happen when trying to link to a "single hostgroup" view from a view showing host rows (where no single host group is availabile).

Instead of showing the crash the GUI is now linking to the target view. Once a user opens up the view, the view is showing a warning message and asks for the missing context information.

To the list of all Werks