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/js/temptyseq.nim

@@ -2,7 +2,7 @@
 
 proc foo =
   var x: seq[int]
-  doAssertRaises(IndexError):
+  doAssertRaises(IndexDefect):
     inc x[0]
 
 foo()