feat: add exception classes and handler class

src/main/java/no/ntnu/idi/stud/savingsapp/exception/auth/InvalidCredentialsException.java

+package no.ntnu.idi.stud.savingsapp.exception.auth;
+
+/**
+ * Exception thrown when authentication fails due to invalid credentials.
+ */
+public class InvalidCredentialsException extends RuntimeException {
+
+  /**
+   * Constructs an InvalidCredentialsException with the default message.
+   */
+  public InvalidCredentialsException() {
+    super("Invalid credentials");
+  }
+}