This won’t throw an error if it’s outside of the source directory though, will it?

This won’t throw an error if it’s outside of the source directory though, will it?

site.in_source_dir essentially joins given path(s) to site.source. It won't throw an error directly.
But yes, an error will be thrown when Ruby attempts to open the file to read it because the file may not exist.

To illustrate with an example, consider the following configuration:

source      : "/usr/projects/blog/john-doe.io"
webmentions :
  templates :
    count   : "../../webmention-templates/count.html"

the paths via using master and this PR-branch will be as follows:

An error will be raised if file at /usr/projects/blog/john-doe.io/webmention-templates/count.html doesn't exist.

Thanks!