Print failure details to STDOUT

[jasonkarns]

Assertions are expected to print failure messages when they fail. As such, STDERR is not the correct place for the expected failure messages to go. This leaves STDERR as the location for error messages from incorrect usage of the helper to go.

Prior art: diff: The diff itself goes to STDOUT (exit 1). When diff is invoked incorrectly, the error message goes to STDERR (exit 2).