Prompt user to install/update external viewer config or gracefully degrade when currently configured viewer is not available, or when importing projects with structure views

Jalview could warn and give the user time to fix their structure viewer preferences when the current configuration requires a particular viewer that is not available on the system, or when importing a project with a structure view that calls for an unavailable viewer.

In Jalview 2.11.2 an error dialog is shown meaning that one has to re-open the project after making sure the external viewers are properly configured.

Ideally, Jalview should provide on the fly configuration so re-opening is not necessary, and/or graceful degradation if a project contains a view produced by an unsupported or unavailable viewer: e.g. opening a Jmol view styled in a similar way to the structure view, and preserving the original structure view in exported projects.