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()

tests/js/tfieldchecks.nim

@@ -24,7 +24,7 @@ var obj = X(f0: "foo", f1: C, f3: 3.14)
 block:
   echo obj.f0
   echo obj.f1
-  doAssertRaises(FieldError): echo obj.f2
+  doAssertRaises(FieldDefect): echo obj.f2
   echo obj.f3
 
 block:
@@ -32,7 +32,7 @@ block:
   echo a0[]
   # let a1 = unsafeAddr(obj.f1)
   # echo a1[]
-  doAssertRaises(FieldError):
+  doAssertRaises(FieldDefect):
     let a2 = addr(obj.f2)
     echo a2[]
   let a3 = addr(obj.f3)