Fix search saving, and add some unit tests

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

test/content/support.js

@@ -114,6 +114,13 @@ function waitForCallback(cb, interval, timeout) {
 	return deferred.promise;
 }
 
+/**
+ * Return a promise for the error thrown by a promise, or false if none
+ */
+function getPromiseError(promise) {
+	return promise.thenReturn(false).catch(e => e);
+}
+
 /**
  * Ensures that the PDF tools are installed, or installs them if not.
  * Returns a promise.