ResponseStatusExceptionFilter: Fix unwrap of JSON message

Reading the entity stream a second time without resetting it was causing
the ObjectMapper to return null then a NullPointerExceptions when
calling get('message') method on it.

Fix the issue by creating the ObjectMapper from the String
representation of the entity stream instead of resetting the stream to
read it a second time. Also check to make sure the returned JsonNode is
not null and that the content is text.

Closes #1222

src/main/java/com/github/dockerjava/jaxrs/filter/ResponseStatusExceptionFilter.java

@@ -10,6 +10,8 @@
 import javax.ws.rs.core.MediaType;
 
 import org.apache.commons.io.IOUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
@@ -31,6 +33,8 @@
  */
 public class ResponseStatusExceptionFilter implements ClientResponseFilter {
 
+    private static final Logger LOG = LoggerFactory.getLogger(ResponseStatusExceptionFilter.class);
+
     @Override
     public void filter(ClientRequestContext requestContext, ClientResponseContext responseContext) throws IOException {
         int status = responseContext.getStatus();
@@ -79,13 +83,19 @@ private String getBodyAsMessage(ClientResponseContext responseContext) {
                 String message = IOUtils.toString(entityStream, charset);
 
                 if (MediaType.APPLICATION_JSON_TYPE.equals(mediaType)) {
-                    ObjectMapper mapper = new ObjectMapper();
-                    JsonNode node = mapper.readTree(entityStream).get("message");
-                    if (node != null) {
-                        message = node.textValue();
+                    try {
+                        JsonNode node = new ObjectMapper().readTree(message);
+                        if (node != null) {
+                            JsonNode messageNode = node.get("message");
+                            if (messageNode != null && messageNode.isTextual()) {
+                                message = messageNode.textValue();
+                            }
+                        }
+                    } catch (IOException e) {
+                        // ignore parsing errors and return the message as is
+                        LOG.debug("Failed to unwrap error message: {}", e.getMessage(), e);
                     }
                 }
-
                 return message;
             } catch (Exception ignored) { }
         }