AMQ-6765 Add trace logging of start failure exception

Log the exception at trace level when start fails.

(cherry picked from commit 56bed30c64)

activemq-http/src/main/java/org/apache/activemq/transport/http/HttpClientTransport.java

@@ -281,6 +281,7 @@ public class HttpClientTransport extends HttpTransportSupport {
             httpClient.execute(httpMethod, new BasicResponseHandler());
             httpClient.execute(optionsMethod, handler);
         } catch(Exception e) {
+            LOG.trace("Error on start: ", e);
             throw new IOException("Failed to perform GET on: " + remoteUrl + " as response was: " + e.getMessage());
         }