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

peru/error.py

@@ -1,3 +1,6 @@
 class PrintableError(Exception):
     def __init__(self, message):
         self.message = message
+
+    def __str__(self):
+        return self.message