Stop and report an error if xcodebuild fails

[johnfairh]

Fixes #643. (and probably the inexplicable stuff in realm/jazzy#1087)

This has happened since SourceKitten started running xcodebuild multiple times with the 'new' build system. If the initial build fails then SourceKitten doesn't notice and can proceed to generate docs from the build manifest that the failing xcodebuild invocation has just created.

There are two things that can happen next:

1. Empty docs are generated. This means xcodebuild has created the build manifest but not populated the Swift response file. Well proven in Empty docs in Xcode 11 (despite latest sourcekitten) #643.
2. Partially complete docs are generated. This time the response file is there, but there is probably a compilation error in one of the files meaning the SourceKit cursorinfo will silently fail in some places. I don't think this is a feature?

I can add a test -- I think would mean adding a failing Xcode project into fixtures.

[jpsim]

It’ll be awkward to test this.

Thanks for the fix and the investigation!