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/destructor/tgotoexceptions5.nim

@@ -13,7 +13,7 @@ proc main() =
   try:
     echo "before"
     raise newException(CatchableError, "foo")
-  except AssertionError:
+  except AssertionDefect:
     echo "caught"
   echo "after"
 
@@ -27,7 +27,7 @@ proc mainB() =
     echo "before"
     raise newException(CatchableError, "foo")
   # except CatchableError: # would work
-  except AssertionError:
+  except AssertionDefect:
     echo "caught"
   except:
     raise