Log rule evaluation errors

Previously, these were only visible if they made it all the way to the
top level of the policy. This hid useful signal about how many PRs
encounter certain types of errors. Log these failures at the info level
so that they appear in logs by default but don't necessarily imply
something is broken in the way a warn/error level log would.

policy/approval/evaluate.go

@@ -63,6 +63,14 @@ func (r *RuleRequirement) Evaluate(ctx context.Context, prctx pull.Context) comm
 	result := r.rule.Evaluate(ctx, prctx)
 	if result.Error == nil {
 		log.Debug().Msgf("rule evaluation resulted in %s:\"%s\"", result.Status, result.StatusDescription)
+	} else {
+		// Log rule evaluations that fail at info level so they appear in logs
+		// by default, but don't log them as warnings or errors since they
+		// don't necessarily break the overall policy (e.g. an 'or' rule can
+		// suppress an error if other members are pending or approved.) Having
+		// this information in logs is useful to understand the rate of
+		// particular types of failures across a policy-bot installation.
+		log.Info().Err(result.Error).Msgf("rule evaluation resulted in error: \"%s\"", result.StatusDescription)
 	}
 
 	return result