Attempt to support invites, working in progress

src/main/groovy/io/kamax/mxisd/exception/MappingAlreadyExistsException.java

@@ -0,0 +1,9 @@
+package io.kamax.mxisd.exception;
+
+public class MappingAlreadyExistsException extends RuntimeException {
+
+    public MappingAlreadyExistsException() {
+        super("A mapping already exists for this 3PID");
+    }
+
+}