The `{{name}}` placeholder could represent different things depending on
the state of the Result, and referring to it as `{{name}}` seems
arbitrary. This commit changes it to `{{subject}}`, which is much more
generic and it describes well what that placeholder can mean.
Currently, the templates that a user provides when running `assert()`
can significantly impact how the message is displayed. Because of this,
the formatters become complex as they all need to handle similar
conditions to format results.
This commit changes this behaviour, letting only the
`InterpolationRenderer` handle the templates. This makes the code
simpler and allows people to use the `InterpolationRenderer` directly,
without needing to figure out how to handle templates. Thinking about it
further, I believe handling templates is a concern for the `Renderer`
anyway, and this will open the way to other improvements using the
renderer.
I also removed the exception that is thrown when the template is not a
string, because I think that after validation has failed, we should not
throw any other exceptions, as that could cause unexpected errors for
users.
