Confusing error text when vat code is malformed

[FUDCo]

Describe the bug
If the source file defining a vat fails to export a buildRootObject function, the error message the kernel prints is vat source bundle lacks (default) setup() function, which is confusing

To Reproduce
PIck a vat. Rename buildRootObject to buildRootOrbject. Run.

Expected behavior
I'd expect an error message more like vat source bundle lacks a buildRootObject() function

[warner]

good point. That error is being generated at

agoric-sdk/packages/SwingSet/src/kernel/vatManager/localVatManager.js

Line 154 in 796dc68

assert(setup, `vat source bundle lacks (default) setup() function`);

, and that code is factored to handle both export function buildRootObject() -style vats and the older export default function setup() style. The presence of a named buildRootObject export should override any default. We have a managerOption flag to enable the use of setup, which is disabled for everything except the comms vat. The particular sequence there is:

• look for buildRootObject, use it if present
• else grab default
  • complain if it's not a function
  • complain if the use of setup is not enabled in the vat options
  • else use it

Maybe that should be refactored to be more like:

• look for buildRootObject, use it if present
• if setup is disabled, complain about the lack of `buildRootObject
• else:
  • grab default
  • complain if it's not a function
  • else use it