fix #12471 (#15901)

* fix #12471

* add testcase

* fix

* test nimdoc is tricky

* Delete t12471.nim

(cherry picked from commit b38f0403fd)

lib/pure/selectors.nim

@@ -47,6 +47,9 @@ when defined(nimdoc):
     Selector*[T] = ref object
       ## An object which holds descriptors to be checked for read/write status
 
+    IOSelectorsException* = object of CatchableError
+      ## Exception that is raised if an IOSelectors error occurs.
+
     Event* {.pure.} = enum
       ## An enum which hold event types
       Read,        ## Descriptor is available for read