Cleaned up feedback (hint) generation

For background, see https://docs.google.com/a/code.org/document/d/1elZH7dGuhq8-s-0-XbtSVDVgcrqkMKkV-I4IxI1NkjQ/edit#heading=h.bdygz33w9e0
This only occurs in Turtle and Maze:
https://github.com/code-dot-org/blockly/blob/master/src/turtle/turtle.js#L625
https://github.com/code-dot-org/blockly/blob/master/src/maze/maze.js#L856

So far, it's always in support of app-specific failures.