Update README
Add error messages for OSStatus error
Add playground
Merge pull request #2 from kishikawakatsumi/error_message

Add error messages for OSStatus error
Merge pull request #3 from kishikawakatsumi/playground

Add playground
Add example project
Improve error message
Merge pull request #4 from kishikawakatsumi/example

Add example project
Merge pull request #5 from kishikawakatsumi/refactor

Improve error message