[19.0][IMP] base_view_inheritance_extension: better error messages for non-locatable nodes

[hbrunn]

before this, having a wrong xpath or similar looked like this:

AttributeError: 'NoneType' object has no attribute 'get'

now it's

Element '<nonexistent />' cannot be located in parent view

View error context:
{'file': '...',
 'line': 1,
 'name': '...',
 'view': ir.ui.view(42,),
 'view.model': False,
 'view.parent': ir.ui.view(4242,),
 'xmlid': '...'}