Cleaned up Error processing

instead of sending errors to a log file, now return any errors, or a
list of errors for the user to handle himself
manually formatted example

gofmt messes up one of the examples
gofmt leaves one tab before every line in a multiline comment
output needs to be indented all the way in in order to be correct,
though
Stop in case of error

• Stop execution of filepath.Walk when there is an error
• Don't append errors after the Walk, because any error returned was
already appended in the Walk