Search condition ids are now indexed from 0, and always saved
contiguously (no more 'fixGaps' option), since they're just in an array
in the API. (They're still returned as an object from
Zotero.Search.prototype.getConditions() because it's easier for the
advanced search window to not have to deal with shifting ids between
saves.)
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
