Hide erros about non-exhaustive pattern matches

There is a problem in sendCommand message that we process some
commands first and compiler can't infer that they are not reachable
in the next clauses. It's possible to ignore the message, but it will
require developer to observe the code each time in order to check
if we have not introduced a new error. Another possibility is to
introduce a datatype for all the commands and try make GHC infter
the unreachability of the clause — probably it's the best scenario
but it's a major change to the library, that can affect downstream
packages. So now we added a error message that works as a comment to
the code and will provide a reasonable error to the user if ever
triggered

Author: qnikst

src/Network/HaskellNet/POP3.hs

@@ -159,6 +159,9 @@ sendCommand conn command =
                          (UIDL msg)   -> "UIDL " ++ maybe "" show msg
                          (APOP usern passw) -> "APOP " ++ usern ++ " " ++
                                              hexDigest (apopKey conn ++ passw)
+                         AUTH _ _ _   -> error "sendCommand: impossible happened AUTH expected to be processed in the preceeding clause"
+                         RETR _       -> error "sendCommand: impossible happened AUTH expected to be processed in the preceeding clause"
+                         TOP _ _      -> error "sendCommand: impossible happened AUTH expected to be processed in the preceeding clause"
 
 user :: POP3Connection -> String -> IO ()
 user conn name = do (resp, _) <- sendCommand conn (USER name)

src/Network/HaskellNet/SMTP/Internal.hs

@@ -302,6 +302,8 @@ sendCommand (SMTPC conn _) meth =
                       NOOP         -> "NOOP"
                       RSET         -> "RSET"
                       QUIT         -> "QUIT"
+                      DATA _       -> error "sendCommand: impossible happened DATA command expected to be processed in thepreceeding clause"
+                      AUTH _ _ _   -> error "sendCommand: impossible happened AUTH command expected to be processed in the preceeding clause"
 
 -- | Sends quit to the server. Connection must be terminated afterwards, i.e. it's not
 -- allowed to issue any command on this connection.