Rework XMPP Error class design

Introduce AbstractError, change 'Conditions' to enums. Because of
AbstractError, it was necessary that PlainStreamElement and
TopLevelStreamElement becomes an interface. Thus the implementation of
TopLevelStreamElement.toString() had to be removed.

This adds

- policy-violation
- unexpected-request

to XMPPError.Condition, and removes the

- payment-required
- remote-server-error
- unexpected-condition
- request-timeout

Conditions

The file transfer code does now no longer throw XMPPErrorExceptions, but
SmackExceptions.

Fixes SMACK-608. Makes it possible to resolves SMACK-386.

smack-extensions/src/main/java/org/jivesoftware/smackx/bytestreams/socks5/Socks5BytestreamRequest.java

@@ -280,7 +280,7 @@ public class Socks5BytestreamRequest implements BytestreamRequest {
      */
     private void cancelRequest() throws XMPPErrorException, NotConnectedException {
         String errorMessage = "Could not establish socket with any provided host";
-        XMPPError error = new XMPPError(XMPPError.Condition.item_not_found, errorMessage);
+        XMPPError error = XMPPError.from(XMPPError.Condition.item_not_found, errorMessage);
         IQ errorIQ = IQ.createErrorResponse(this.bytestreamRequest, error);
         this.manager.getConnection().sendPacket(errorIQ);
         throw new XMPPErrorException(errorMessage, error);