Error when make yarn start on /server

Error when make $ yarn start
on /fullstack-graphql-airbnb-clone/packages/server

```
yarn run v1.19.1
warning package.json: No license field
$ cross-env NODE_ENV=development nodemon --exec ts-node src/index.ts
[nodemon] 1.17.5
[nodemon] to restart at any time, enter `rs`
[nodemon] watching: *.*
[nodemon] starting `ts-node src/index.ts`
query: START TRANSACTION
query: SELECT * FROM current_schema()
query: SELECT * FROM "information_schema"."tables" WHERE ("table_schema" = 'public' AND "table_name" = 'listings') OR ("table_schema" = 'public' AND "table_name" = 'users') OR ("table_schema" = 'public' AND "table_name" = 'messages')
query: SELECT *, "udt_name"::"regtype" AS "regtype" FROM "information_schema"."columns" WHERE ("table_schema" = 'public' AND "table_name" = 'listings') OR ("table_schema" = 'public' AND "table_name" = 'users') OR ("table_schema" = 'public' AND "table_name" = 'messages')
query: SELECT "ns"."nspname" AS "table_schema", "t"."relname" AS "table_name", "cnst"."conname" AS "constraint_name", "cnst"."consrc" AS "expression", CASE "cnst"."contype" WHEN 'p' THEN 'PRIMARY' WHEN 'u' THEN 'UNIQUE' WHEN 'c' THEN 'CHECK' END AS "constraint_type", "a"."attname" AS "column_name" FROM "pg_constraint" "cnst" INNER JOIN "pg_class" "t" ON "t"."oid" = "cnst"."conrelid" INNER JOIN "pg_namespace" "ns" ON "ns"."oid" = "cnst"."connamespace" INNER JOIN "pg_attribute" "a" ON "a"."attrelid" = "cnst"."conrelid" AND "a"."attnum" = ANY ("cnst"."conkey") WHERE "t"."relkind" = 'r' AND (("ns"."nspname" = 'public' AND "t"."relname" = 'listings') OR ("ns"."nspname" = 'public' AND "t"."relname" = 'users') OR ("ns"."nspname" = 'public' AND "t"."relname" = 'messages'))
query: SELECT "ns"."nspname" AS "table_schema", "t"."relname" AS "table_name", "i"."relname" AS "constraint_name", "a"."attname" AS "column_name", CASE "ix"."indisunique" WHEN 't' THEN 'TRUE' ELSE'FALSE' END AS "is_unique", pg_get_expr("ix"."indpred", "ix"."indrelid") AS "condition" FROM "pg_class" "t" INNER JOIN "pg_index" "ix" ON "ix"."indrelid" = "t"."oid" INNER JOIN "pg_attribute" "a" ON "a"."attrelid" = "t"."oid" AND "a"."attnum" = ANY ("ix"."indkey") INNER JOIN "pg_namespace" "ns" ON "ns"."oid" = "t"."relnamespace" INNER JOIN "pg_class" "i" ON "i"."oid" = "ix"."indexrelid" LEFT JOIN "pg_constraint" "cnst" ON "cnst"."conname" = "i"."relname" WHERE "t"."relkind" = 'r' AND "cnst"."contype" IS NULL AND (("ns"."nspname" = 'public' AND "t"."relname" = 'listings') OR ("ns"."nspname" = 'public' AND "t"."relname" = 'users') OR ("ns"."nspname" = 'public' AND "t"."relname" = 'messages'))
query: SELECT "con"."conname" AS "constraint_name", "con"."nspname" AS "table_schema", "con"."relname" AS "table_name", "att2"."attname" AS "column_name", "ns"."nspname" AS "referenced_table_schema", "cl"."relname" AS "referenced_table_name", "att"."attname" AS "referenced_column_name", "con"."confdeltype" AS "on_delete", "con"."confupdtype" AS "on_update" FROM ( SELECT UNNEST ("con1"."conkey") AS "parent", UNNEST ("con1"."confkey") AS "child", "con1"."confrelid", "con1"."conrelid", "con1"."conname", "con1"."contype", "ns"."nspname", "cl"."relname", CASE "con1"."confdeltype" WHEN 'a' THEN 'NO ACTION' WHEN 'r' THEN 'RESTRICT' WHEN 'c' THEN 'CASCADE' WHEN 'n' THEN 'SET NULL' WHEN 'd' THEN 'SET DEFAULT' END as "confdeltype", CASE "con1"."confupdtype" WHEN 'a' THEN 'NO ACTION' WHEN 'r' THEN 'RESTRICT' WHEN 'c' THEN 'CASCADE' WHEN 'n' THEN 'SET NULL' WHEN 'd' THEN 'SET DEFAULT' END as "confupdtype" FROM "pg_class" "cl" INNER JOIN "pg_namespace" "ns" ON "cl"."relnamespace" = "ns"."oid" INNER JOIN "pg_constraint" "con1" ON "con1"."conrelid" = "cl"."oid" WHERE "con1"."contype" = 'f' AND (("ns"."nspname" = 'public' AND "cl"."relname" = 'listings') OR ("ns"."nspname" = 'public' AND "cl"."relname" = 'users') OR ("ns"."nspname" = 'public' AND "cl"."relname" = 'messages')) ) "con" INNER JOIN "pg_attribute" "att" ON "att"."attrelid" = "con"."confrelid" AND "att"."attnum" = "con"."child" INNER JOIN "pg_class" "cl" ON "cl"."oid" = "con"."confrelid" INNER JOIN "pg_namespace" "ns" ON "cl"."relnamespace" = "ns"."oid" INNER JOIN "pg_attribute" "att2" ON "att2"."attrelid" = "con"."conrelid" AND "att2"."attnum" = "con"."parent"
query: COMMIT
query: SELECT "Listing"."id" AS "Listing_id", "Listing"."name" AS "Listing_name", "Listing"."category" AS "Listing_category", "Listing"."pictureUrl" AS "Listing_pictureUrl", "Listing"."description" AS "Listing_description", "Listing"."price" AS "Listing_price", "Listing"."beds" AS "Listing_beds", "Listing"."guests" AS "Listing_guests", "Listing"."latitude" AS "Listing_latitude", "Listing"."longitude" AS "Listing_longitude", "Listing"."amenities" AS "Listing_amenities", "Listing"."userId" AS "Listing_userId" FROM "listings" "Listing"
Error: listen EADDRINUSE :::4000
at Object._errnoException (util.js:1022:11)
at _exceptionWithHostPort (util.js:1044:20)
at Server.setupListenHandle [as _listen2] (net.js:1367:14)
at listenInCluster (net.js:1408:12)
at Server.listen (net.js:1492:7)
at /home/hax0r/Desktop/fullstack-graphql-airbnb-clone/node_modules/graphql-yoga/src/index.ts:368:22
at new Promise ()
at GraphQLServer.start (/home/hax0r/Desktop/fullstack-graphql-airbnb-clone/node_modules/graphql-yoga/src/index.ts:366:12)
at Object. (/home/hax0r/Desktop/fullstack-graphql-airbnb-clone/packages/server/src/startServer.ts:116:28)
at Generator.next ()
at fulfilled (/home/hax0r/Desktop/fullstack-graphql-airbnb-clone/packages/server/src/startServer.ts:4:58)
at
at process._tickDomainCallback (internal/process/next_tick.js:228:7)
[nodemon] app crashed - waiting for file changes before starting...

```

Start with the `yarn start` command in `/packages/server` and inspect the server startup at `packages/server/src/startServer.ts:116`, where the stack trace reports `EADDRINUSE` on port 4000. Reproduce the startup failure and determine the expected behavior when that port is already occupied; done should be verified by running the server successfully without the reported crash.

Written by the indexing model from the issue text.