lua/fiber: improve error message raised on invalid slice

Error messages raised when an invalid slice is specified are confusing:

  tarantool> fiber = require('fiber')
  ---
  ...

  tarantool> fiber.set_max_slice('foo')
  ---
  - error: slice must be a table or a number
  ...

  tarantool> fiber.set_max_slice({})
  ---
  - error: 'bad argument #3 to ''?'' (number expected, got nil)'
  ...

Let's change the error message to "slice must be a number or a table
{warn = <number>, err = <number>}".

Follow-up #6085

NO_DOC=undocumented
NO_CHANGELOG=unreleased