Quality of life improvements

[mikew]

Really loving this library, paired with coc.nvim we've got great coverage with linters, formatters, and language servers.

When playing with a few linters, I noticed some issues:

• not all fixers support stdin/stdout communication
• when a formatter fails, the file is replaced with the errors output
• there should be a way to run linters / formatters only when certain files exist

This PR handles those situations.

[mikew]

I've addressed those changes

[iamcco]

Thanks for your work!