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/stdlib/tbitops.nim

@@ -266,7 +266,7 @@ block: # not ready for vm because exception is compile error
     var i = 32
     v.setBit(i)
     doAssert false
-  except RangeError:
+  except RangeDefect:
     discard
   except:
     doAssert false