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/std/wrapnils.nim

@@ -64,7 +64,7 @@ template `[]`*[I](a: Wrapnil, i: I): untyped =
   ## See top-level example.
   let a1 = a # to avoid double evaluations
   if a1.validImpl:
-    # correctly will raise IndexError if a is valid but wraps an empty container
+    # correctly will raise IndexDefect if a is valid but wraps an empty container
     wrapnil(a1.valueImpl[i])
   else:
     default(Wrapnil[type(a1.valueImpl[i])])