fix(rsc): add getEntrySource assertion error message

[hi-ogawa]

Description

Extracted from #632

Probably still cryptic but better

### before
assert(
    typeof input === 'object' &&
      !Array.isArray(input) &&
      name in input &&
      typeof input[name] === 'string')

### after
AssertionError [ERR_ASSERTION]: [vite-rsc:getEntrySource] expected 'build.rollupOptions.input' to be an object with a 'index' property that is a string, but got "/home/hiroshi/code/others/vite-plugin-react/packages/plugin-rsc/examples/react-router/cf/entry.ssr.tsx"