add __str__ for PrintableError
add __str__ for PrintableError

Summary: This makes it easier to read plugin errors that happen during tests.

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D119

oconnor663/peru,olson-sean-k/peru,enzochiau/peru,buildinspace/peru,oconnor663/peru,scalp42/peru,buildinspace/peru,nivertech/peru,ierceg/peru,ierceg/peru,nivertech/peru,enzochiau/peru,olson-sean-k/peru,scalp42/peru