fix(publish): error on missing name field

[marvinhagemeister]

This PR improves the error output on publish when the name filed is missing:

{
  "exports": "./mod.ts",
  "version": "0.0.1"
}

Before:

deno publish --dry-run
error: You did not specify an entrypoint in file:///Users/marvinh/dev/test/deno-pkg-timers/deno.json. Add `exports` mapping in the configuration file, eg:
{
  "name": "@scope/name",
  "version": "0.0.0",
  "exports": "<path_to_entrypoint>"
}

After:

deno publish --dry-run
error: Missing 'name' field in 'file:///Users/marvinh/dev/test/deno-pkg-timers/deno.json'.

Fixes #27116

[dsherret]

LGTM! Seems I improved the exports error message too much lol