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/assert/tassert_c.nim

@@ -33,7 +33,7 @@ try:
   proc foo() =
     assert(false)
   foo()
-except AssertionError:
+except AssertionDefect:
   let e = getCurrentException()
   let trace = e.getStackTrace
   if tmatch(trace, expected): echo true else: echo "wrong trace:\n" & trace