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/effects/teffects7.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "can raise an unlisted exception: ref FloatingPointError"
+  errormsg: "can raise an unlisted exception: ref FloatingPointDefect"
   line: 10
 """
 
@@ -7,7 +7,7 @@ proc foo() {.raises: [].} =
   try:
     discard
   except KeyError:
-    raise newException(FloatingPointError, "foo")
+    raise newException(FloatingPointDefect, "foo")
   except Exception:
     discard