[Validator] Error when xcodebuild fails

I gave up trying to write a test for this. Either merge without test, someone add a test and merge, or reject this patch.

Here's a base for anyone willing to add tests:

```diff
+      it 'errors when xcodebuild exits with non 0 exit code' do
+        Process::Status.any_instance.stubs(:success?).returns(false)
+
+        validator = Validator.new(podspec_path, SourcesManager.master.map(&:url))
+        validator.stubs(:check_file_patterns)
+        validator.stubs(:validate_url)
+        validator.validate
+
+        validator.results.count.should == 1
+        validator.result_type.should == :error
+      end
+
```
xcodebuild's output is already printed in verbose?

```ruby
UI.puts 'xcodebuild clean build -target Pods' if config.verbose?
```