Clearly log if only parts of a buffer are sent to some linter

[kaste]

Related: #1492

If we send the contents of the buffer to a specific linter partially (e.g. the js parts of a html file), we should log that. E.g. 'eslint' is linting 'my-file.html' can be somewhat misleading. And $ cat my-file.html | eslint ... is just wrong.