Handling of symbology load error on project load

[ccrook]

This is a proposed patch to change the behaviour of QGis if a layer fails to load because it cannot process the symbology definition in the project file (eg because a renderer plugin is missing).

The current behaviour is to fail to load the layer. The attached patch changes this to instead use a default symbology.

Neither is ideal ... I think it would be better to load a default symbology and report to the user that the original symbology could not be loaded - preferably with a reason why. However that raises a bigger question about how to handle errors when loading the project file.

At the moment if the symbology doesn't load, the layer fails, and the user is presented with the bad layers handler. This allows them to reselect the file used by the layer. However this is really confusing if the file is correct and exists, but the symbology cannot be loaded ... it confused me anyway!

With this patch the layer loads, but the symbology reverts to a default definition - not what the user would expect, but the best that can be done.