Builder::generate should not return Result<_, ()>

[MikuroXina]

According to Clippy, it is not useful that Builder::generate (Bindings::generate internally) returns a Result<_, ()> because it carries no information about what went wrong. Currently the information is printed to stderr by eprintln!, but it is hard to handle an error programatically. I think that we should change it into a custom error type.

warning: this returns a `Result<_, ()>`
Warning:     --> src/lib.rs:1447:5
     |
1447 |     pub fn generate(mut self) -> Result<Bindings, ()> {
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: `#[warn(clippy::result_unit_err)]` on by default
     = help: use a custom `Error` type instead
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#result_unit_err

This introduces a breaking change, so we'll first add new improved method then deprecate the old one.