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/tstdlib_various.nim

@@ -169,7 +169,7 @@ block tsegfaults:
         raise newException(ValueError, "not a crash")
       except ValueError:
         discard
-    except NilAccessError:
+    except NilAccessDefect:
       echo "caught a crash!"
   for i in 0..5:
     main()
@@ -212,7 +212,7 @@ block tsplit2:
   try:
     discard "hello".split("")
     echo "false"
-  except AssertionError:
+  except AssertionDefect:
     echo "true"