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/tut3.rst

@@ -271,7 +271,7 @@ written.
 
     result = quote do:
       if not `arg`:
-        raise newException(AssertionError,$`lhs` & `op` & $`rhs`)
+        raise newException(AssertionDefect,$`lhs` & `op` & $`rhs`)
 
   let a = 1
   let b = 2
@@ -287,7 +287,7 @@ used to get this output.
 
 .. code-block:: nim
   if not (a != b):
-    raise newException(AssertionError, $a & " != " & $b)
+    raise newException(AssertionDefect, $a & " != " & $b)
 
 With Power Comes Responsibility
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