when xcodebuild fails, print the same error message xcodebuild would … 

Previously, we'd tell you nothing.  Now we'll show something like...

```
=== CLEAN ===

  xcodebuild clean ProjectsWithDifferentSDKs
    OSXLibrary / OSXLibrary (Debug)
      ✓ Check dependencies (50 ms)
      ✓ Remove OSXLibrary.build (1 ms)
      ✓ Remove OSXLibrary.dylib (1 ms)
      ✓ Remove OSXLibrary-Prefix.pch.pth (1 ms)

    iOSLibrary / iOSLibrary (Debug)

  xcodebuild clean Tests
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Failed to build workspace Tests with scheme Tests.
Reason: Scheme "Tests" is not configured for running.
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

** CLEAN FAILED ** (986 ms)
```