Run method use channels for errors and outputs, but also use a fmt.Fprintf(os.Stderr

We could check what could be done to simplify things. Ideas such as:

• check of the method signature shouldn't be updated to return an error. Handling error with printing is strange to me,it should be moved to higher level.
• check if other errors currently send to outputCh or errCh shouldn't also be refactored

Originally posted by @ccoVeille in #60 (comment)