Handle econnrefused gracefully

When remote HTTP server is down, in h2_connection:init/1 Transport:connect will return econnrefused. As this call is handled in h2_connection:init/1, current code returns {stop,econnrefused}. Although this is handled gracefully by clients, we do see an CRASH report saying that:

`[error] CRASH REPORT Process <0.2985.0> with 0 neighbours exited with reason: econnrefused in gen_statem:init_result/6 line 728`

gen_statem:init origin routine would have called exit with a reason code other than normal and this triggers the CRASH report. If the gen_statem module has returned {stop, {shutdown,Reason}}, the gen_statem process would call Module:terminate/1 and exit appropriately without a crash report.

src/h2_connection.erl

@@ -193,10 +193,8 @@ init({client, Transport, Host, Port, SSLOptions, Http2Settings, ConnectionSettin
              handshake,
              send_settings(Http2Settings, InitialState),
              4500};
-        {error, econnrefused} ->
-            ignore;
         {error, Reason} ->
-            {stop, Reason}
+            {stop, {shutdown,Reason}}
     end;
 init({client_ssl_upgrade, Host, Port, InitialMessage, SSLOptions, Http2Settings, ConnectionSettings}) ->
     case gen_tcp:connect(Host, Port, [{active, false}]) of
@@ -221,10 +219,8 @@ init({client_ssl_upgrade, Host, Port, InitialMessage, SSLOptions, Http2Settings,
                      handshake,
                      send_settings(Http2Settings, InitialState),
                      4500};
-                {error, econnrefused} ->
-                    ignore;
                 {error, Reason} ->
-                    {stop, Reason}
+                    {stop, {shutdown,Reason}}
             end;
         {error, Reason} ->
             {stop, Reason}