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.

compiler/semfold.nim

@@ -645,9 +645,9 @@ proc getConstExpr(m: PSym, n: PNode; g: ModuleGraph): PNode =
         discard
       else:
         result = magicCall(m, n, g)
-    except OverflowError:
+    except OverflowDefect:
       localError(g.config, n.info, "over- or underflow")
-    except DivByZeroError:
+    except DivByZeroDefect:
       localError(g.config, n.info, "division by zero")
   of nkAddr:
     var a = getConstExpr(m, n[0], g)