Forced to use exit error codes for normal tasks

[aminya]

#50 added a new change that does not make sense for normal outputs when someone is not running Shell commands. Rust has rich error-handling features, which are much better than exit codes.

Also, Output::error method is changed, which is a breaking change for all the users of Dagrs. We have to revert this change by adding a new status field like ErrWithExitCode

Originally posted by @aminya in #50 (comment)