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.

lib/pure/httpclient.nim

@@ -26,11 +26,11 @@
 ##
 ## .. code-block:: Nim
 ## import asyncdispatch, httpclient
-## 
+##
 ## proc asyncProc(): Future[string] {.async.} =
 ##   var client = newAsyncHttpClient()
 ##   return await client.getContent("http://example.com")
-## 
+##
 ## echo waitFor asyncProc()
 ##
 ## The functionality implemented by ``HttpClient`` and ``AsyncHttpClient``
@@ -225,7 +225,7 @@ type
     bodyStream*: FutureStream[string]
 
 proc code*(response: Response | AsyncResponse): HttpCode
-           {.raises: [ValueError, OverflowError].} =
+           {.raises: [ValueError, OverflowDefect].} =
   ## Retrieves the specified response's ``HttpCode``.
   ##
   ## Raises a ``ValueError`` if the response's ``status`` does not have a