ropensci/targets: New helpers, better relaying in Target Markdown, and better exception handling

targets 0.7.0 Bug fixes Ensure error = "continue" does not cause errored targets to have NULL values. Relay output and messages in Target Markdown interactive mode (using the R/default knitr engine). New features Expose the poll_connection, stdout, and stderr arguments of callr::r_bg() in tar_watch() (@mpadge). Add new helper functions to list targets in each progress category: tar_started(), tar_skipped(), tar_built(), tar_canceled(), and tar_errored(). Add new helper functions tar_interactive(), tar_noninteractive(), and tar_toggle() to differentially suppress code in non-interactive and interactive mode in Target Markdown (#607, @33Vito). Enhancements Handle future errors within targets (#570, @stuvet). Handle storage errors within targets (#571, @stuvet). In Target Markdown in non-interactive mode, suppress messages if the message knitr chunk option is FALSE (#574, @jmbuhr). In Target Markdown, if tar_interactive is not set, choose interactive vs non-interactive mode based on isTRUE(getOption("knitr.in.progress")) instead of interactive(). Convert errors loading dependencies into errors running targets (@stuvet)