feat: created group not found exception.

src/main/java/no/ntnu/idi/stud/savingsapp/exception/goal/GroupNotFoundException.java

+package no.ntnu.idi.stud.savingsapp.exception.goal;
+
+public final class GroupNotFoundException extends RuntimeException{
+
+  /**
+   * Constructs a GroupNotFoundException with default message.
+   */
+  public GroupNotFoundException() { super("Group not found");}
+
+  /**
+   * Constructs a GroupNotFoundException with custom message.
+   *
+   * @param string the custom exception message
+   */
+  public GroupNotFoundException(String string) {super(string);}
+}