chore: implement str representation for serializable error info

Summary:
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