Use Exception.getString in HostAddress

instead of getMessage(), because some Exceptions, e.g. Android's
NetworkOnMainThreadException, will return null on
getMessage(). Exception.toString() does what we want, i.e. returns
- the exception class name
- and the return value of getLocalizedMessage()

smack-core/src/main/java/org/jivesoftware/smack/util/dns/HostAddress.java

@@ -109,13 +109,9 @@ public class HostAddress {
     }
 
     public String getErrorMessage() {
-        String error;
         if (exception == null) {
-            error = "No error logged";
+            return "No error logged";
         }
-        else {
-            error = exception.getMessage();
-        }
-        return toString() + " Exception: " + error;
+        return "'" + toString() + "' failed because " + exception.toString();
     }
 }

smack-core/src/test/java/org/jivesoftware/smack/SmackExceptionTest.java

@@ -41,7 +41,7 @@ public class SmackExceptionTest {
 
         ConnectionException connectionException = ConnectionException.from(failedAddresses);
         String message = connectionException.getMessage();
-        assertEquals("The following addresses failed: foo.bar.example:1234 Exception: Failed for some reason, barz.example:5678 Exception: Failed for some other reason",
+        assertEquals("The following addresses failed: 'foo.bar.example:1234' failed because java.lang.Exception: Failed for some reason, 'barz.example:5678' failed because java.lang.Exception: Failed for some other reason",
                         message);
     }