Better error message when running xcodebuild fails

Because this:
```
2013-05-23 22:56:40.209 xctool[40141:f07] Error running xcodebuild: dyld: could not load inserted library: /Users/0xced/Library/Developer/Xcode/DerivedData/xctool-ddidqmmbossgpqamltbydmrbuquk/Build/Products/Debug/xcodebuild-fastsettings-shim.dylib
```
is much more useful than this:
```
Assertion failed: (settings.count == 1), function -[XcodeSubjectInfo populate], file /Users/0xced/Projects/xctool/xctool/xctool/XcodeSubjectInfo.m, line 730.
```