Add Adfruit_MQTT::connectErrorString() method

This easily allows printing error messages without having to hard-code
return values in the sketch and makes the example sketches a bit more
concise.

Adafruit_MQTT.cpp

@@ -140,6 +140,19 @@ int8_t Adafruit_MQTT::connect() {
   return 0;
 }
 
+const __FlashStringHelper* Adafruit_MQTT::connectErrorString(int8_t code) {
+   switch (code) {
+      case 1: return F("Wrong protocol");
+      case 2: return F("ID rejected");
+      case 3: return F("Server unavail");
+      case 4: return F("Bad user/pass");
+      case 5: return F("Not authed");
+      case 6: return F("Failed to subscribe");
+      case -1: return F("Connection failed");
+      default: return F("Unknown error");
+   }
+}
+
 bool Adafruit_MQTT::publish(const char *topic, const char *data, uint8_t qos) {
   // Construct and send publish packet.
   uint8_t len = publishPacket(buffer, topic, data, qos);

Adafruit_MQTT.h

@@ -105,8 +105,16 @@ class Adafruit_MQTT {
   //    4 = Bad username or password
   //    5 = Not authenticated
   //    6 = Failed to subscribe
+  // Use connectErrorString() to get a printable string version of the
+  // error.
   int8_t connect();
 
+  // Return a printable string version of the error code returned by
+  // connect(). This returns a __FlashStringHelper*, which points to a
+  // string stored in flash, but can be directly passed to e.g.
+  // Serial.println without any further processing.
+  const __FlashStringHelper* connectErrorString(int8_t code);
+
   // Disconnect from the MQTT server.  Returns true if disconnected, false
   // otherwise.
   virtual bool disconnect() = 0;  // Subclasses need to fill this in!

examples/mqtt_cc3k/mqtt_cc3k.ino

@@ -152,18 +152,9 @@ void MQTT_connect() {
   Serial.print("Connecting to MQTT... ");
 
   while ((ret = mqtt.connect()) != 0) { // connect will return 0 for connected
-       switch (ret) {
-          case 1: Serial.println("Wrong protocol"); break;
-          case 2: Serial.println("ID rejected"); break;
-          case 3: Serial.println("Server unavailable"); break;
-          case 4: Serial.println("Bad user/password"); break;
-          case 5: Serial.println("Not authenticated"); break;
-          case 6: Serial.println("Failed to subscribe"); break;
-          default:
-            Serial.println(F("Connection failed"));
+       Serial.println(mqtt.connectErrorString(ret));
+       if (ret < 0)
             CC3000connect(WLAN_SSID, WLAN_PASS, WLAN_SECURITY);  // y0w, lets connect to wifi again
-            break;
-       }
        Serial.println("Retrying MQTT connection in 5 seconds...");
        mqtt.disconnect();
        delay(5000);  // wait 5 seconds

examples/mqtt_esp8266/mqtt_esp8266.ino

@@ -137,17 +137,7 @@ void MQTT_connect() {
   Serial.print("Connecting to MQTT... ");
 
   while ((ret = mqtt.connect()) != 0) { // connect will return 0 for connected
-       switch (ret) {
-          case 1: Serial.println("Wrong protocol"); break;
-          case 2: Serial.println("ID rejected"); break;
-          case 3: Serial.println("Server unavailable"); break;
-          case 4: Serial.println("Bad user/password"); break;
-          case 5: Serial.println("Not authenticated"); break;
-          case 6: Serial.println("Failed to subscribe"); break;
-          default: Serial.print("Couldn't connect to server, code: ");
-                   Serial.println(ret);
-                   break;
-       }
+       Serial.println(mqtt.connectErrorString(ret));
        Serial.println("Retrying MQTT connection in 5 seconds...");
        mqtt.disconnect();
        delay(5000);  // wait 5 seconds

examples/mqtt_fona/mqtt_fona.ino

@@ -122,18 +122,7 @@ void loop() {
     Serial.println(F("Connecting to MQTT..."));
     int8_t ret, retries = 5;
     while (retries && (ret = mqtt.connect()) != 0) {
-       switch (ret) {
-          case 1: Serial.println(F("Wrong protocol")); break;
-          case 2: Serial.println(F("ID rejected")); break;
-          case 3: Serial.println(F("Server unavail")); break;
-          case 4: Serial.println(F("Bad user/pass")); break;
-          case 5: Serial.println(F("Not authed")); break;
-          case 6: Serial.println(F("Failed to subscribe")); break;
-          default: {
-            Serial.println(F("Connection failed"));
-            break;
-          }
-       }
+       Serial.println(mqtt.connectErrorString(ret));
        Serial.println(F("Retrying MQTT connection"));
        retries--;
        if (retries == 0) halt("Resetting system");