errors: Add a __repr__.

This makes the errors easier to understand in test failures.

conservancy_beancount/plugin/errors.py

@@ -20,6 +20,13 @@ class _BaseError(Exception):
         self.entry = entry
         self.source = entry.meta if source is None else source
 
+    def __repr__(self):
+        return "{clsname}<{source[filename]}:{source[lineno]}: {message}>".format(
+            clsname=type(self).__name__,
+            message=self.message,
+            source=self.source,
+        )
+
 
 class InvalidMetadataError(_BaseError):
     def __init__(self, txn, post, key, value=None, source=None):