Allow to error one stream and publish another

[pauby]

Currently, if you have 2 streams where one fails and other is updated, you can either use 'ignore' to publish updated stream and ignore error from failed one or throw an exception and fail the whole update process for that package without publishing stream which got updated. However there is no way to "throw" an exception for one stream without failing whole package.

So I think, there should be an option to put StreamsErrors along with Streams in hashtable that au_GetLatest returns. It would contain catched exceptions thrown by failed streams (or possibly just error messages, but I think passing whole exception will allow more control) for streams that failed and also increment error count returned by vars like $global:info.error_count.total, so we could fail AppVeyor build on that kind of "partial" package error too.

Issue migrated from majkinetor/au#190