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.

doc/lib.rst

@@ -451,7 +451,7 @@ Miscellaneous
   This module implements a simple logger.
 
 * `segfaults <segfaults.html>`_
-  Turns access violations or segfaults into a ``NilAccessError`` exception.
+  Turns access violations or segfaults into a ``NilAccessDefect`` exception.
 
 * `sugar <sugar.html>`_
   This module implements nice syntactic sugar based on Nim's macro system.