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/exception/texceptions.nim

@@ -89,10 +89,10 @@ block:
       result.add(1212)
     try:
       try:
-        raise newException(AssertionError, "a")
+        raise newException(AssertionDefect, "a")
       finally:
         result.add(42)
-    except AssertionError:
+    except AssertionDefect:
       result.add(99)
     finally:
       result.add(10)
@@ -104,7 +104,7 @@ block:
       result.add(-1)
     except ValueError:
       result.add(-1)
-    except IndexError:
+    except IndexDefect:
       result.add(2)
     except:
       result.add(3)

tests/exception/twrongexc.nim

@@ -4,5 +4,5 @@ discard """
 """
 try:
   raise newException(ValueError, "")
-except OverflowError:
+except OverflowDefect:
   echo("Error caught")