improving the error message we show when failing to query testable settings

We used to show ...

```
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Could not get build settings. Output of 'xcodebuid -showBuildSettings' was:
(stdout): Build settings from command line:
    OBJROOT = /Users/fpotter/Library/Developer/Xcode/DerivedData/TestWorkspace-Library-dfkuuumeqgksbpctikbmroulmipg/Build/Intermediates
    PLATFORM_NAME = iphonesimulator
    SDKROOT = iphonesimulator7.0
    SHARED_PRECOMPS_DIR = /Users/fpotter/Library/Developer/Xcode/DerivedData/TestWorkspace-Library-dfkuuumeqgksbpctikbmroulmipg/Build/Intermediates/PrecompiledHeaders
    SYMROOT = /Users/fpotter/Library/Developer/Xcode/DerivedData/TestWorkspace-Library-dfkuuumeqgksbpctikbmroulmipg/Build/Products

(stderr): xcodebuild: error: The project 'xctool/xctool-tests/TestData/TestWorkspace-Library/TestProject-Library/TestProject-Library.xcodeproj' does not contain a target named 'TestProject-LibraryTests823123'.
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```

But, let's show ...

```
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Unble to read build settings for target 'TestProject-LibraryTests823123'.  It's likely that the scheme references a non-existent target.

Output from `xcodebuild -showBuildSettings`:

Build settings from command line:
    OBJROOT = /Users/fpotter/Library/Developer/Xcode/DerivedData/TestWorkspace-Library-dfkuuumeqgksbpctikbmroulmipg/Build/Intermediates
    PLATFORM_NAME = iphonesimulator
    SDKROOT = iphonesimulator7.0
    SHARED_PRECOMPS_DIR = /Users/fpotter/Library/Developer/Xcode/DerivedData/TestWorkspace-Library-dfkuuumeqgksbpctikbmroulmipg/Build/Intermediates/PrecompiledHeaders
    SYMROOT = /Users/fpotter/Library/Developer/Xcode/DerivedData/TestWorkspace-Library-dfkuuumeqgksbpctikbmroulmipg/Build/Products

xcodebuild: error: The project 'xctool/xctool-tests/TestData/TestWorkspace-Library/TestProject-Library/TestProject-Library.xcodeproj' does not contain a target named 'TestProject-LibraryTests823123'.
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```