Added error handling for not found objects

oparl-server/src/main/java/de/twomartens/oparlservice/entity/ErrorObject.java

@@ -0,0 +1,23 @@
+package de.twomartens.oparlservice.entity;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import io.swagger.v3.oas.annotations.media.Schema;
+import lombok.*;
+
+@Builder
+@Getter
+@ToString
+@EqualsAndHashCode
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@NoArgsConstructor(force = true, access = AccessLevel.PRIVATE)
+@JsonInclude(JsonInclude.Include.NON_NULL)
+public class ErrorObject {
+    @Schema(description = "URL to type specification")
+    private final String type = "https://schema.oparl.org/1.1/Error";
+
+    @Schema(description = "message for display to the user")
+    private final String message;
+
+    @Schema(description = "additional information about the error")
+    private final String debug;
+}