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/niminaction/Chapter2/various2.nim

@@ -179,13 +179,13 @@ for i in list4.low .. list4.high:
   echo(list4[i])
 
 var list5: seq[int] = @[]
-doAssertRaises(IndexError):
+doAssertRaises(IndexDefect):
   list5[0] = 1
 
 list5.add(1)
 
 assert list5[0] == 1
-doAssertRaises(IndexError):
+doAssertRaises(IndexDefect):
   echo list5[42]
 
 # -- Errata: var list: seq[int]; echo(list[0]). This now creates an exception,