[GREEN] Add custom exceptions for user authentication errors

src/main/java/com/safetypin/authentication/exception/InvalidCredentialsException.java

+package com.safetypin.authentication.exception;
+
+public class InvalidCredentialsException extends RuntimeException {
+    public InvalidCredentialsException(String message) {
+        super(message);
+    }
+}

src/main/java/com/safetypin/authentication/exception/UserAlreadyExistsException.java

+package com.safetypin.authentication.exception;
+
+public class UserAlreadyExistsException extends RuntimeException {
+    public UserAlreadyExistsException(String message) {
+        super(message);
+    }
+}