Set exception cause when re-raising IOException

This allows to execute custom logic in AsyncHttpResponseHandler.onFailure
for specific errors, e.g. to display a nicer error message to the user when
there is a DNS resolution failure, instead of leaking a useless and verbose
UnknownHostException in the UI.

library/src/main/java/com/loopj/android/http/AsyncHttpRequest.java

@@ -180,7 +180,7 @@ private void makeRequestWithRetries() throws IOException {
                     // switching between WI-FI and mobile data networks can cause a retry which then results in an UnknownHostException
                     // while the WI-FI is initialising. The retry logic will be invoked here, if this is NOT the first retry
                     // (to assist in genuine cases of unknown host) which seems better than outright failure
-                    cause = new IOException("UnknownHostException exception: " + e.getMessage());
+                    cause = new IOException("UnknownHostException exception: " + e.getMessage(), e);
                     retry = (executionCount > 0) && retryHandler.retryRequest(e, ++executionCount, context);
                 } catch (NullPointerException e) {
                     // there's a bug in HttpClient 4.0.x that on some occasions causes
@@ -203,7 +203,7 @@ private void makeRequestWithRetries() throws IOException {
         } catch (Exception e) {
             // catch anything else to ensure failure message is propagated
             AsyncHttpClient.log.e("AsyncHttpRequest", "Unhandled exception origin cause", e);
-            cause = new IOException("Unhandled exception: " + e.getMessage());
+            cause = new IOException("Unhandled exception: " + e.getMessage(), cause);
         }
 
         // cleaned up to throw IOException