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/stdlib/tunittest.nim

@@ -56,7 +56,7 @@ proc defectiveRobot() =
   of 3: raise newException(IOError, "I can't do that Dave.")
   else: assert 2 + 2 == 5
 test "unittest expect":
-  expect IOError, OSError, ValueError, AssertionError:
+  expect IOError, OSError, ValueError, AssertionDefect:
     defectiveRobot()
 
 var