[BUG] failing eval_code breaks the shiny app

[gogonzo]

When eval_code fails quosure is not returned anymore. It means that reactive which fails return error and breaks the whole app. What we actually need is to still return the quosure and keep the error messages (and warnings) per code element to print the proper diagnostics.
If we decide to drop diagnostics feature some tryCatch is still needed, because reactives still should return quosure and instead of error message we would like to use validate(q3) in shiny apps.

q1 <- new_quosure()
q2 <- eval_code(q1, iris2 <- iris)

q3 <- eval_code(q2, stop("elo"))
q3[["iris2"]]

r1 <- reactive(new_quosure())
r2 <- reactive(eval_code(r1(), iris2 <- iris))

r3 <- reactive(r2(), stop("elo"))

get_code(r3()) # get_code fails as r3 doesn't return quosure anymore

Final acceptance criteria not yet formulated