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/async/tupcoming_async.nim

@@ -45,7 +45,7 @@ when defined(upcoming):
     poll()
 
   proc eventTest5298() =
-    # Event must raise `AssertionError` if event was unregistered twice.
+    # Event must raise `AssertionDefect` if event was unregistered twice.
     # Issue #5298.
     let e = newAsyncEvent()
     var eventReceived = false
@@ -57,7 +57,7 @@ when defined(upcoming):
       poll()
     try:
       e.unregister()
-    except AssertionError:
+    except AssertionDefect:
       discard
     e.close()