Switch AppSearchBatchResult from Throwable to error code and message.

Based on API review feedback:
> A map of Throwable objects is very unusual. Couldn't you return a
> structure of error code and reason string? If you know of specific
> error types, it's better to be explicit rather than have to parse
> just an exception string.

Bug: 143789408
Test: atest CtsAppSearchTestCases FrameworksCoreTests:android.app.appsearch FrameworksServicesTests:com.android.server.appsearch.impl
Change-Id: I8d3df1a176fd275db54783b387602ceb7b0d280a