Nicer arguments and messages

Bug fixes

• In tar_load(), subset metadata to avoid accidental attempts to load global objects in tidyselect calls.
• Do not register a pattern as running unless an actual branch is about to start (#304).
• Use a name spec in vctrs::vec_c() (#320, @joelnitta).

New features

• Add a new names argument to tar_objects() and tar_workspaces() with tidyselect functionality.
• Record info on the main process (PID, R version, targets version) in _targets/meta/process and write new functions tar_process() and tar_pid() to retrieve the data (#291, #292).
• Add a new targets_only argument to tar_meta().
• Add new functions tar_helper() and tar_helper_raw() to write general-purpose R scripts, using tidy evaluation for as a template mechanism (#290, #291, #292, #306).
• Export functions to check the existence of various pieces of local storage: tar_exist_meta(), tar_exist_objects(), tar_exist_progress(), tar_exist_progress(), tar_exist_script() (#310).
• Add a new supervise argument to tar_watch().
• Add a new complete_only argument to tar_meta() to optionally return only complete rows (no NA values).
• Catch callr errors and refer users to the debugging chapter of the manual.

Enhancements

• Improve error messages of invalid arguments (#298, @brunocarlin). Removes partial argument matching in most cases.
• By default, locally enable crayon if an only if the calling process is interactive (#302, @ginolhac). Can still be disabled with options(crayon.enabled = FALSE) in _targets.R.
• Improve error handling and message for format = "url" when the HTTP response status code is not 200 (#303, @petrbouchal).
• Add more extras packages to tar_renv() (to support tar_watch()).
• Show informative message instead of error in tar_watch() if _targets.R does not exist.
• Clear up the documentation of the names argument of tar_load() (#314, @jameelalsalam).
• Do not override nobody in custom curl handles (#315, @riazarbi).
• Rename "running" to "started" in the progress metadata. This avoids the implicit claim that targets is somehow actively monitoring each job, e.g. through a connection or heartbeat (#318).
• Set errormode = "warn" in getVDigest() for files to work around eddelbuettel/digest#49 for network drives on Windows. targets already runs those file checks anyway. (#316, @boshek).
• If a package fails to load, print the library paths targets tried to load from.