[Bf-codereview] First set of fixes (issue 7193061)

[lukas.toenne at gmail.com]

Merged trunk revisions up to r54370. Also includes an important fix to
avoid crashes when accessing node from API with undefined types, e.g. if
a loading nodes from .blend whose types are defined by a script that has
not been run. Undefined types can cause error reports and possibly py
exceptions, but should never crash Blender.

http://codereview.appspot.com/7193061/