Introduce a MessageContext class for more flexibility

This change will allow to pass message dependencies from the client
validators to the ValidationManager.

briar-api/src/org/briarproject/api/sync/InvalidMessageException.java

@@ -0,0 +1,20 @@
+package org.briarproject.api.sync;
+
+import java.io.IOException;
+
+/** An exception that indicates an invalid message. */
+public class InvalidMessageException extends IOException {
+
+	public InvalidMessageException() {
+		super();
+	}
+
+	public InvalidMessageException(String str) {
+		super(str);
+	}
+
+	public InvalidMessageException(Throwable t) {
+		super(t);
+	}
+
+}