fix: improve error when cannot read from enum table

[benjie]

Fixes #650

New error follows:

A serious error occurred when building the initial schema. Exiting because `retryOnInitFail` is not set. Error details:

Error: Introspection could not read from enum table "app_public"."enum_test", perhaps you need to grant access:

  GRANT USAGE ON SCHEMA "app_public" TO "graphile_starter_authenticator";
  GRANT SELECT ON "app_public"."enum_test" TO "graphile_starter_authenticator";

Original error: permission denied for schema app_public

    at /home/benjie/Dev/graphile/graphile-engine/packages/graphile-build-pg/src/plugins/PgIntrospectionPlugin.js:676:25
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async Promise.all (index 193)
    at /home/benjie/Dev/graphile/graphile-engine/packages/graphile-build-pg/src/plugins/PgIntrospectionPlugin.js:557:11
    at withPgClient (/home/benjie/Dev/graphile/graphile-engine/packages/graphile-build-pg/src/withPgClient.js:93:12)
    at introspect (/home/benjie/Dev/graphile/graphile-engine/packages/graphile-build-pg/src/plugins/PgIntrospectionPlugin.js:491:7)
    at PgIntrospectionPlugin (/home/benjie/Dev/graphile/graphile-engine/packages/graphile-build-pg/src/plugins/PgIntrospectionPlugin.js:1073:39)
    at Object.getBuilder (/home/benjie/Dev/graphile/graphile-engine/packages/graphile-build/src/index.js:64:5)
    at Object.exports.createPostGraphileSchema (/home/benjie/Dev/graphile/graphile-engine/packages/postgraphile-core/src/index.ts:438:19)
    at createGqlSchema (/home/benjie/Dev/graphile/graphile-engine/postgraphile/src/postgraphile/postgraphile.ts:118:23)