Add error notification in the IDE when generator start fails.

[ReinhardKeil]

The Generator interface checks all projects that are in a solution (because they all share the same generator project - see csolution list generators *.csolution.yml` -v. However if there is an error in a cproject file that is not part of the current context set, there is no visible error notification. The user therefore believes the generator interface is broken.

Workaround is to open the Output window.