feat(core): Create improved error-handling infrastructure

This commit introduces a new graphql-code-generator plugin which generates error classes and supporting code to enable error result union types as described in vendure-ecommerce#437

package.json

@@ -13,7 +13,7 @@
     "docs:watch": "concurrently --restart-tries 5 -n docgen,hugo,webpack -c green,blue,cyan \"yarn generate-graphql-docs && yarn generate-typescript-docs -w\" \"cd docs && hugo server\" \"cd docs && yarn webpack -w\"",
     "docs:build": "yarn generate-graphql-docs && yarn generate-typescript-docs && cd docs && yarn webpack --prod && node build.js && hugo",
     "docs:deploy": "cd docs && yarn && cd .. && yarn docs:build",
-    "codegen": "ts-node scripts/codegen/generate-graphql-types.ts",
+    "codegen": "tsc -p scripts/codegen/plugins && ts-node scripts/codegen/generate-graphql-types.ts",
     "generate-typescript-docs": "ts-node scripts/docs/generate-typescript-docs.ts",
     "generate-graphql-docs": "ts-node scripts/docs/generate-graphql-docs.ts --api=shop && ts-node scripts/docs/generate-graphql-docs.ts --api=admin",
     "version": "yarn check-imports && yarn build && yarn generate-changelog && git add CHANGELOG.md && git add */version.ts",
@@ -29,12 +29,12 @@
   "devDependencies": {
     "@commitlint/cli": "^8.2.0",
     "@commitlint/config-conventional": "^8.2.0",
-    "@graphql-codegen/add": "1.13.1",
-    "@graphql-codegen/cli": "1.13.1",
-    "@graphql-codegen/fragment-matcher": "1.13.1",
-    "@graphql-codegen/typescript": "1.13.1",
-    "@graphql-codegen/typescript-compatibility": "1.13.1",
-    "@graphql-codegen/typescript-operations": "1.13.1",
+    "@graphql-codegen/add": "2.0.1",
+    "@graphql-codegen/cli": "1.17.8",
+    "@graphql-codegen/fragment-matcher": "1.17.8",
+    "@graphql-codegen/typescript": "1.17.9",
+    "@graphql-codegen/typescript-compatibility": "2.0.0",
+    "@graphql-codegen/typescript-operations": "1.17.8",
     "@types/graphql": "^14.0.5",
     "@types/jest": "^25.1.4",
     "@types/klaw-sync": "^6.0.0",