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).