Allow passing back nan in language agnostic version as a failed trial

[madeline-scyphers]

When constructing output.json, if your metric(s) = nan, then one or more metric failed to return a result, which could be a way to make writing an output.json with a failed trial even simpler. Instead of having to write something like:

res <- hartman6(X)
if (!is.na(res)) {
    out_data <- list(metric=res)
} else {
    out_data <- list(trial_status=unbox("FAILED"))
}
write(toJSON(list(metric=res)), file.path(trial_dir, "output.json"))

you could simplify it to this:

write(toJSON(list(metric=hartman6(unlist(data)))), file.path(trial_dir, "output.json"))

or something like that.

[madeline-scyphers]

Added in #168