Make broken expectations catchable by try()

[jcheng5]

Currently, broken expectations break rmarkdown document rendering
even if knitr chunk option error=TRUE. It also breaks the evaluate
package's ability to capture errors.

This PR is just a starting point for discussion, if you like it I can add
tests.

[hadley]

This seems right to me,

[hadley]

Hmmmm, this does break a test:

test_that("multiple failures captured even when looking for errors", {
  f <- function() {
    expect_true(FALSE)
    expect_false(TRUE)
  }

  reporter <- with_reporter("silent", {
    test_that("", expect_error(f(), NA))
  })
  expect_equal(length(reporter$expectations()), 3)
})

Presumably because expect_error() shouldn't terminate.

[hadley]

Probably because of this tryCatch():

  error <- tryCatch(
    {
      object
      NULL
    },
    error = function(e) {
      e
    }
  )

[hadley]

So I here I think a condition of class exception (but not error) should resume. I think this means we need some unholy hybrid of withCallingHandlers() and tryCatch() or @lionel-'s rlang::with_handlers()

[hadley]

Same basic problem in #618