Add AuthException

It will be used when an unauthorized request is made
or we fail to authenticate from headers.

app/src/main/java/org/vss/api/AbstractVssApi.java

@@ -7,6 +7,7 @@
 import org.vss.ErrorCode;
 import org.vss.ErrorResponse;
 import org.vss.KVStore;
+import org.vss.exception.AuthException;
 import org.vss.exception.ConflictException;
 import org.vss.exception.NoSuchKeyException;
 
@@ -39,6 +40,9 @@ Response toErrorResponse(Exception e) {
     } else if (e instanceof NoSuchKeyException) {
       errorCode = ErrorCode.NO_SUCH_KEY_EXCEPTION;
       statusCode = 404;
+    } else if (e instanceof AuthException) {
+      errorCode = ErrorCode.AUTH_EXCEPTION;
+      statusCode = 401;
     } else {
       errorCode = ErrorCode.INTERNAL_SERVER_EXCEPTION;
       statusCode = 500;

app/src/main/java/org/vss/exception/AuthException.java

@@ -0,0 +1,7 @@
+package org.vss.exception;
+
+public class AuthException extends RuntimeException {
+  public AuthException(String message) {
+    super(message);
+  }
+}

app/src/main/proto/vss.proto

@@ -268,7 +268,7 @@ enum ErrorCode {
 
   // Used when the request contains mismatched version (either key or global)
   // in `PutObjectRequest`. For more info refer `PutObjectRequest`.
-  CONFLICT_EXCEPTION= 1;
+  CONFLICT_EXCEPTION = 1;
 
   // Used in the following cases:
   //   - The request was missing a required argument.
@@ -282,6 +282,9 @@ enum ErrorCode {
 
   // Used when the specified `key` in a `GetObjectRequest` does not exist.
   NO_SUCH_KEY_EXCEPTION = 4;
+
+  // Used when authentication fails or in case of an unauthorized request.
+  AUTH_EXCEPTION = 5;
 }
 
 // Represents a key-value pair to be stored or retrieved.