Rewrite expectation style

[hadley]

Drop use of expect() in favour of an if statement that branches to fail(). Every expectation should call succeed() before returning the value.

In most cases, will want to treat fail() like an early return, i.e. instead of

expect_no_success <- function(expr) {
  exp <- capture_success(expr)

  if (!is.null(exp)) {
    fail("Expectation succeeded")
  } else {
    succeed()
  }
  invisible(NULL)
}

Should be

expect_no_success <- function(expr) {
  exp <- capture_success(expr)
  if (!is.null(exp)) {
    fail("Expectation succeeded")
  }
  
  succeed()
  invisible(NULL)
}

And then update the documentation in the custom expectations vignette.

[hadley]

Superseded by #2113