make tests green again

examples/tunit.nim

@@ -42,6 +42,6 @@ test "arithmetic failure":
   expect(ArithmeticError):
     err()
 
-  expect(ArithmeticError, SystemError):
+  expect(ArithmeticError, CatchableError):
     discard foo()
 

tests/exprs/tstmtexprs.nim

@@ -87,7 +87,7 @@ proc parseResponse(): JsonNode =
     var excMsg = key & "("
     if (var n=result["key2"]; n != nil):
       excMsg &= n.str
-    raise newException(SystemError, excMsg)
+    raise newException(CatchableError, excMsg)