Implement device linking

src/main/java/org/asamk/signal/UserAlreadyExists.java

@@ -0,0 +1,19 @@
+package org.asamk.signal;
+
+public class UserAlreadyExists extends Exception {
+    private String username;
+    private String fileName;
+
+    public UserAlreadyExists(String username, String fileName) {
+        this.username = username;
+        this.fileName = fileName;
+    }
+
+    public String getUsername() {
+        return username;
+    }
+
+    public String getFileName() {
+        return fileName;
+    }
+}