Handle errors once

[evanp]

We should have a way to pass errors from a topic to a single error handler per batch, or a single global handler.

The way we're handling errors in 1.x, you get a lot of duplication if you have multiple tests:

vows.describe("handle error once")
  .addBatch({
    "When we read a file": {
      topic() {
        fs.readFile("/tmp/somefile.txt", "utf8", this.callback)
      },
      "it is the right data type": (err, data) => {
        assert.ifError(err)
        assert.isString(data)
      },
      "it is the right size": (err, data) => {
        assert.ifError(err)
        assert.lengthOf(data, 42)
      },
      "it has the right contents": (err, data) => {
        assert.ifError(err)
        assert.match(data, /some contents/)
      }
    }
  })
  .export(module)

It would be nice if, when you define an error handler for the batch, that would get called for any errors, and tests would only be run if there is no error.

vows.describe("handle error once")
  .addBatch({
    "When we read a file": {
      topic() {
        fs.readFile("/tmp/somefile.txt", "utf8", this.callback)
      },
      error(err) {
        console.error(err)
      },
      "it is the right data type": (data) => {
        assert.isString(data)
      },
      "it is the right size": (data) => {
        assert.lengthOf(data, 42)
      },
      "it has the right contents": (data) => {
        assert.match(data, /some contents/)
      }
    }
  })
  .export(module)

We could even have a mechanism for declaring a global error handler.

vows.options.errorHandler = (err) => {
  console.error(err)
}

vows.describe("default error handler")
  .addBatch({
    "When we read a file": {
      topic() {
        fs.readFile("/tmp/somefile.txt", "utf8", this.callback)
      },
      "it is the right data type": (data) => {
        assert.isString(data)
      },
      "it is the right size": (data) => {
        assert.lengthOf(data, 42)
      },
      "it has the right contents": (data) => {
        assert.match(data, /some contents/)
      }
    }
  })
  .export(module)

This would only handle errors from the topic:

1. If the topic function throws an Error
2. If this.callback is called with a first argument that's truthy
3. If the topic returns a Promise that reject()s

If any of the tests throw an error, that's treated as a test failure.