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-core/src/test/java/org/jivesoftware/smack/packet/StreamErrorTest.java

@@ -20,6 +20,7 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.fail;
 
+import org.jivesoftware.smack.packet.StreamError.Condition;
 import org.jivesoftware.smack.util.PacketParserUtils;
 import org.junit.Test;
 import org.xmlpull.v1.XmlPullParser;
@@ -44,7 +45,7 @@ public class StreamErrorTest {
             fail(e.getMessage());
         }
         assertNotNull(error);
-        assertEquals("conflict", error.getCode());
+        assertEquals(Condition.conflict, error.getCondition());
     }
 
     @Test
@@ -68,8 +69,8 @@ public class StreamErrorTest {
             fail(e.getMessage());
         }
         assertNotNull(error);
-        assertEquals("conflict", error.getCode());
-        assertEquals("Replaced by new connection", error.getText());
+        assertEquals(Condition.conflict, error.getCondition());
+        assertEquals("Replaced by new connection", error.getDescriptiveText());
     }
 
     @Test
@@ -84,7 +85,7 @@ public class StreamErrorTest {
                 "<text xml:lang='' xmlns='urn:ietf:params:xml:ns:xmpp-streams'>" +
                     "Replaced by new connection" +
                 "</text>" +
-                "<appSpecificElement>" +
+                "<appSpecificElement xmlns='myns'>" +
                     "Text contents of application-specific condition element: Foo Bar" +
                 "</appSpecificElement>" +
                 "</stream:error>" +
@@ -96,10 +97,10 @@ public class StreamErrorTest {
             fail(e.getMessage());
         }
         assertNotNull(error);
-        assertEquals("conflict", error.getCode());
-        assertEquals("Replaced by new connection", error.getText());
-        // As of now, Smack ignores application-specific condition elements, so we don't
-        // test them.
+        assertEquals(Condition.conflict, error.getCondition());
+        assertEquals("Replaced by new connection", error.getDescriptiveText());
+        PacketExtension appSpecificElement = error.getExtension("appSpecificElement", "myns");
+        assertNotNull(appSpecificElement);
     }
 
 }