Also:
- Return an object from `Zotero.Search.prototype.getConditions()`
instead of an array.
- Add support function `getPromiseError(promise)` to return the error
thrown from a chain of promises, or false if none. (We could make an
`assert.throwsAsync()`, but this allows testing of various properties
such as `.name`, which even the built-in `assert.throws()` can't
test.)
- Clarify some search save errors
Also some support code that was useful here and will probably be
useful for other tests. This is a pretty complicated thing to test, but
it seems to work.
