Fix crash, improve error messages when opening an invalid provider

- When trying to eg. open a GTFS provider file, TimetableMate crashed,
because it tried to access an invalid pointer to the Project object,
as returned by TimetableMate::openProject()
- There was no error message shown when opening an invalid provider
because the message gets emitted from the Project constructor,
therefore it cannot be connected in time
- Add a lastError() function to Project, to workaround the problem above
- Do not call loadProject() in the Project constructor, needs to be
explicitely called now