WIP: Update .throw and add .error

[meeber]

DO NOT MERGE; THIS IS A WORK IN PROGRESS!!

I've been doing a lot of work on #930. This turned out to be a lot harder than expected; .throw couldn't be updated to simply call .error internally due to the difference in failed assertion messages between the two. I suspect this is the same kind of issue causing problems in plugins like chai-as-promised when they try to adapt .throw.

The more I looked at our current .throw implementation, the more I realized that the complexity wasn't because of negation as we originally thought but rather because of how the failed assertion messages were being composed. As a result, this PR turned into a pretty big refactor of .throw to reduce complexity. This also required changing some of .throw's failed assertion messages to be more consistent.

Some notes/thoughts:

• This PR currently only adds .error to the BDD interface. I'll add the Assert interface later.
• Is changing a failed assertion message considered a breaking change? On the one hand, it's likely to break the tests of any plugins that do string matching against failed assertion messages. On the other hand, it's unlikely to break the actual functionality of the plugin from the end user's perspective, or any of the end user's tests. I'm currently learning toward "no".
• The matchError and decribeExpectedError helper functions should probably be moved to the check-error module.
• Part of what .throw does is assert that a function throws, without taking into account what it throws. This means it will work if the target function throws something besides an Error object (e.g., a string, a number, null, or even undefined). I think it's important that the .throw logic retain this base functionality. However, when it comes to adding arguments to the .throw function to perform additional assertions on the value that was thrown, I think we should consider requiring that the object thrown be an Error object. Currently, the behavior of the various check-error functions is a little sketchy if the object being tested isn't an Error. This problem carries over into the matchError helper function as well.

[meeber]

Didn't mean to close this. I think I broke it; the reopen button is grayed out for me. I'll create a new PR later if it doesn't magically fix itself.