APIClient: Fix error handling for POST if API call fails
APIClient: Fix error handling for POST if API call fails

Currently, we have a scary looking dtest failure when attempting to force flush a

  Nodetool command '/data/jenkins/workspace/scylla-1.3-dtest/label/monster/mode/release/smp/1/scylla/resources/cassandra/bin/nodetool -h localhost -p 7100 flush' failed; exit status: 2; stderr: Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF8
  error: javax.ws.rs.ProcessingException (no security manager: RMI class loader disabled)
  -- StackTrace --
  java.lang.ClassNotFoundException: javax.ws.rs.ProcessingException (no security manager: RMI class loader disabled)
          at sun.rmi.server.LoaderHandler.loadClass(LoaderHandler.java:396)
          at sun.rmi.server.LoaderHandler.loadClass(LoaderHandler.java:186)
          at java.rmi.server.RMIClassLoader$2.loadClass(RMIClassLoader.java:637)
          at java.rmi.server.RMIClassLoader.loadClass(RMIClassLoader.java:264)
          at sun.rmi.server.MarshalInputStream.resolveClass(MarshalInputStream.java:219)
          at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1620)
          at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1521)
          at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1781)
          at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1353)
          at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:2018)
          at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1942)
          at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1808)
          at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1353)
          at java.io.ObjectInputStream.readObject(ObjectInputStream.java:373)
          at sun.rmi.transport.StreamRemoteCall.executeCall(StreamRemoteCall.java:245)
          at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:162)
          at com.sun.jmx.remote.internal.PRef.invoke(Unknown Source)
          at javax.management.remote.rmi.RMIConnectionImpl_Stub.invoke(Unknown Source)
          at javax.management.remote.rmi.RMIConnector$RemoteMBeanServerConnection.invoke(RMIConnector.java:1020)
          at javax.management.MBeanServerInvocationHandler.invoke(MBeanServerInvocationHandler.java:298)
          at com.sun.proxy.$Proxy7.forceKeyspaceFlush(Unknown Source)
          at org.apache.cassandra.tools.NodeProbe.forceKeyspaceFlush(NodeProbe.java:290)
          at org.apache.cassandra.tools.NodeTool$Flush.execute(NodeTool.java:1227)
          at org.apache.cassandra.tools.NodeTool$NodeToolCmd.run(NodeTool.java:288)
          at org.apache.cassandra.tools.NodeTool.main(NodeTool.java:202)

The problem is rather innocent: the API call fails and we leak
javax.ws.rs.ProcessingException, which is not available in nodetool's
classpath. In fact, we already fixed the problem for GETs in commit
02e0598 ("APIClient: Fix error handling if connection to API server
fails") so do the same thing for POSTs.
Message-Id: <acfe95984e33bdf7cebbb3c9972ebee02b6f123f@scylladb.com>

scylladb/scylla-jmx,scylladb/scylla-jmx,scylladb/scylla-jmx