Remove dead event, clarify Rmd/HTML docs

[trestletech]

It appears that exercise_error in the docs is no longer valid. That string doesn't exist in the code -- at least not anymore. When I submit an invalid exercise, I get an excercise_submission event with an error field.

e.g. if I submit stop("what") in an exercise, I get this event:

com.rstudio.primers.02-vis-basics (1): jeff
event: exercise_submission
$label
[1] "mpg0"
$code
[1] "x <- 1\nx\nstop(\"what\")\n\nflarg"
$output
<div class="alert alert-danger" role="alert">what</div>
$error_message
[1] "what"
$checked
[1] FALSE
$feedback
NULL

Also fixes #327