chore: implement str representation for serializable error info
Rather than calling .to_string() (which we might forget when implementing
debugging), implement the string representation of the object so that printing
will always give the human readable string for a serializable error.
Test Plan: print object in console, now shows pretty string
Reviewers: dgibson, sidkmenon, alangenfeld
Reviewed By: sidkmenon
Differential Revision: https://dagster.phacility.com/D8634