Expose UndefinedFactError and add test

[rdvoskin-recur]

PROBLEM

• UndefinedFactError is not exported which makes it hard to catch
• UndefinedFactError.code value is not exported with makes it hard to detect
• No tests exist for UndefinedFactError

SOLUTION

• Export UndefinedFactError and UndefinedFactErrorCode
• Add unit test
• Add babel plugin for Error to make instanceof work properly with babel

[rdvoskin-recur]

@CacheControl I look forward to your review. This is an excellent library, and I want to do my part to support it :)