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/system/channels.nim

@@ -347,7 +347,7 @@ template lockChannel(q, action): untyped =
 
 proc sendImpl(q: PRawChannel, typ: PNimType, msg: pointer, noBlock: bool): bool =
   if q.mask == ChannelDeadMask:
-    sysFatal(DeadThreadError, "cannot send message; thread died")
+    sysFatal(DeadThreadDefect, "cannot send message; thread died")
   acquireSys(q.lock)
   if q.maxItems > 0:
     # Wait until count is less than maxItems