Make clap usable, add warnings on missing fact files

[lqd]

The ./inputs/clap-rs dataset doesn't work on master, and fails with the rather unhelpful error

ERROR 2019-09-22T00:39:26Z: polonius::cli: `./inputs/clap-rs/app-parser-{{impl}}-add_defaults/`: No such file or directory (os error 2)

even though this directory does exist. The problem is that some of the fact files are missing.

This PR adds a log for when facts files are missing, but still continues execution, defaulting to an empty relation. clap generates these now:

ERROR 2019-09-22T00:42:02Z: polonius::tab_delim: Error opening file './inputs/clap-rs/app-parser-{{impl}}-add_defaults/child.facts': No such file or directory (os error 2). Defaulting to empty relation
ERROR 2019-09-22T00:42:02Z: polonius::tab_delim: Error opening file './inputs/clap-rs/app-parser-{{impl}}-add_defaults/path_belongs_to_var.facts': No such file or directory (os error 2). Defaulting to empty relation
ERROR 2019-09-22T00:42:02Z: polonius::tab_delim: Error opening file './inputs/clap-rs/app-parser-{{impl}}-add_defaults/initialized_at.facts': No such file or directory (os error 2). Defaulting to empty relation
ERROR 2019-09-22T00:42:02Z: polonius::tab_delim: Error opening file './inputs/clap-rs/app-parser-{{impl}}-add_defaults/moved_out_at.facts': No such file or directory (os error 2). Defaulting to empty relation
ERROR 2019-09-22T00:42:02Z: polonius::tab_delim: Error opening file './inputs/clap-rs/app-parser-{{impl}}-add_defaults/path_accessed_at.facts': No such file or directory (os error 2). Defaulting to empty relation

I thought of adding empty files to the dataset to silence the warnings, but leaving them on will remind us to regenerate the clap dataset with these new relations correctly exported.