Return code of NSTasks are never checked

[guillaumealgis]

Some plugins fail to install properly (SublimeShortcuts-ObjC for example) because the xcodebuild install command fails.

They are still reported as installed because in the build process the PluginName.xcplugin folder is created in ~/Library/Application Support/Developer/Shared/Xcode/Plug-ins and that's all we are checking to determine if a plugin is installed.

We should use -[NSTask terminationStatus] to ensure the build exited with a 0 error code.