Allow specification of custom error type

[TedDriggs]

Crates may already have an error into which they'd like to feed builder failures. Add #[builder(build_fn(error = "..."))] to allow specifying that, and document the contract for that custom error type.

[andy128k]

Alternatively a simpler solution is possible. Just allow to make build() method and error type private.

#[derive(Builder)]
#[builder(error(private))]
#[builder(build_fn(name = "build_impl", private))]
struct Lorem {
    pub ipsum: u8,
}

impl LoremBuilder {
    pub fn build(&self) -> Result<Lorem, CustomError> {
        self.build_impl()
            .map_err(|private_lorem_builder_error| CustomError::new(... private_lorem_builder_error...))
    }
}