Enriching IOException

[kazu-yamamoto]

The current IOException thrown by network is too poor.
For instance, bind does not tell which port is already used.
Let's enrich IOException.

The definition is here:

data IOException
 = IOError {
     ioe_handle   :: Maybe Handle,   -- the handle used by the action flagging
                                     -- the error.
     ioe_type     :: IOErrorType,    -- what it was.
     ioe_location :: String,         -- location.
     ioe_description :: String,      -- error type specific information.
     ioe_errno    :: Maybe CInt,     -- errno leading to this error, if any.
     ioe_filename :: Maybe FilePath  -- filename the error is related to.
   }

Show is here:

instance Show IOException where
    showsPrec p (IOError hdl iot loc s _ fn) =
      (case fn of
         Nothing -> case hdl of
                        Nothing -> id
                        Just h  -> showsPrec p h . showString ": "
         Just name -> showString name . showString ": ") .
      (case loc of
         "" -> id
         _  -> showString loc . showString ": ") .
      showsPrec p iot .
      (case s of
         "" -> id
         _  -> showString " (" . showString s . showString ")")

ieo_errno is set but it's not displayed.
So, ioe_description should include it.