Error -> Defect for defects (#13908)

* Error -> Defect for defects

The distinction between Error and Defect is subjective,
context-dependent and somewhat arbitrary, so when looking at an
exception, it's hard to guess what it is - this happens often when
looking at a `raises` list _without_ opening the corresponding
definition and digging through layers of inheritance.

With the help of a little consistency in naming, it's at least possible
to start disentangling the two error types and the standard lib can set
a good example here.

tests/objects/tobj_asgn_dont_slice.nim

@@ -1,5 +1,5 @@
 discard """
-  outputsub: '''ObjectAssignmentError'''
+  outputsub: '''ObjectAssignmentDefect'''
   exitcode: "1"
 """