Clean up error returns from functions

Currently, the code base has many different ways for functions to report errors:

- `chrome.runtime.lastError`
- `true` for success, stringy error message for failure
- truthy object for success, stringy error message for failure
- `null` for failure, other for success

Regularize these!

Maybe https://github.com/alleycat-at-git/monad/blob/master/js/src/option.js .

Start by inventorying the functions that use the listed error-return conventions and reading the surrounding callers to understand their expectations. Compare the alternatives mentioned in the issue before deciding whether one convention can cover the codebase. Done means the error-reporting behavior is regularized consistently across the affected functions and their callers.

Written by the indexing model from the issue text.