moved missing multipart accept header error into errors.js

index.js

@@ -38,7 +38,8 @@ const {
   MER_ERR_GQL_VALIDATION,
   MER_ERR_INVALID_OPTS,
   MER_ERR_METHOD_NOT_ALLOWED,
-  MER_ERR_INVALID_METHOD
+  MER_ERR_INVALID_METHOD,
+  MER_ERR_INVALID_MULTIPART_ACCEPT_HEADER
 } = require('./lib/errors')
 const { Hooks, assignLifeCycleHooksToContext } = require('./lib/hooks')
 const { kLoaders, kFactory, kSubscriptionFactory, kHooks } = require('./lib/symbols')
@@ -577,12 +578,7 @@ const plugin = fp(async function (app, opts) {
       ) {
         // The client ran an operation that would yield multiple parts, but didn't
         // specify `accept: multipart/mixed`. We return an error.
-        throw new Error(
-          'Server received an operation that uses incremental delivery ' +
-          '(@defer or @stream), but the client does not accept multipart/mixed ' +
-          'HTTP responses. To enable incremental delivery support, add the HTTP ' +
-          "header 'Accept: multipart/mixed; deferSpec=20220824'."
-        )
+        throw new MER_ERR_INVALID_MULTIPART_ACCEPT_HEADER()
       }
 
       reply.header('content-type', 'multipart/mixed; boundary="-"; deferSpec=20220824')

lib/errors.js

@@ -139,6 +139,10 @@ const errors = {
     'Method not allowed',
     405
   ),
+  MER_ERR_INVALID_MULTIPART_ACCEPT_HEADER: createError(
+    'MER_ERR_INVALID_MULTIPART_ACCEPT_HEADER',
+    'Server received an operation that uses incremental delivery (@defer or @stream), but the client does not accept multipart/mixed HTTP responses. To enable incremental delivery support, add the HTTP header "Accept: multipart/mixed; deferSpec=20220824".'
+  ),
   /**
    * General graphql errors
    */

test/defer.js

@@ -86,11 +86,11 @@ if (canUseIncrementalExecution) {
       })
 
       t.match(res, {
-        statusCode: 200,
+        statusCode: 500,
         body: JSON.stringify({
           data: null,
           errors: [{
-            message: "Server received an operation that uses incremental delivery (@defer or @stream), but the client does not accept multipart/mixed HTTP responses. To enable incremental delivery support, add the HTTP header 'Accept: multipart/mixed; deferSpec=20220824'."
+            message: 'Server received an operation that uses incremental delivery (@defer or @stream), but the client does not accept multipart/mixed HTTP responses. To enable incremental delivery support, add the HTTP header "Accept: multipart/mixed; deferSpec=20220824".'
           }]
         })
       })