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/excpt.nim

@@ -483,7 +483,7 @@ proc raiseException(e: sink(ref Exception), ename: cstring) {.compilerRtl.} =
 
 proc reraiseException() {.compilerRtl.} =
   if currException == nil:
-    sysFatal(ReraiseError, "no exception to reraise")
+    sysFatal(ReraiseDefect, "no exception to reraise")
   else:
     when gotoBasedExceptions:
       inc nimInErrorMode