improve error handling (#101)

* inventory of error handling problems and types

* Change SMP protocol errors syntax

* connection errors in agent protocol (ERR CONN), STORE error -> AGENT error

* include exception in SEInternal error

* add MESSAGE errors, remove CRYPTO and SIZE errors

* agent protocol SYNTAX and AGENT errors

* BROKER errors

* group all client command (and agent response) errors

* BROKER TRANSPORT error

* simplify Client

* clean up

* transport errors

* simplify client

* parse / serialize agent errors

* differentiate crypto errors

* update errors.md

* make agent and SMP protocol errors consistent, simplify

* update doc

* test: parse / serialize protocol errors with QuickCheck

* add String to internal error

* exponential back-off when retrying to send HELLO

* refactor Client.hs

* replace fold with recursion in startTCPClient

* fail test if server did not start, refactor

* test: wait till TCP server stops

* test: refactor waiting for server to stop

* test: fail with error if server did not start/stop

src/Simplex/Messaging/Server/QueueStore/STM.hs

@@ -32,7 +32,7 @@ instance MonadQueueStore QueueStore STM where
   addQueue store rKey ids@(rId, sId) = do
     cs@QueueStoreData {queues, senders} <- readTVar store
     if M.member rId queues || M.member sId senders
-      then return $ Left DUPLICATE
+      then return $ Left DUPLICATE_
       else do
         writeTVar store $
           cs