Instead, panic, which only fails one request instead of sending the
server into an infinite loop.

This shouldn't ever happen in practice since error pages are tested in
CI, but it's nice just in case.

The panic looks like this:

```
2021/06/21 00:05:07 [ERROR] docs_rs::web::page::web_page: called ctry!() on an `Err` value: Error { kind: Msg("Failed to render 'error.html'"), source: Some(Error { kind: Msg("Variable `oops` not found in context while rendering 'error.html'"), source: None }) }
note: while attempting to fetch the route Url { generic_url: "http://localhost:3000/not_here" }
   4: docs_rs::web::page::web_page::WebPage::into_response<docs_rs::web::ErrorPage>
             at src\web\page\web_page.rs:74
   5: docs_rs::web::error::{{impl}}::handle
             at src\web\error.rs:78
   6: iron::middleware::{{impl}}::handle
             at C:\Users\Joshua Nelson\.cargo\registry\src\github.com-1ecc6299db9ec823\iron-0.6.1\src\middleware\mod.rs:395
  10: docs_rs::web::{{impl}}::handle::{{closure}}
             at src\web\mod.rs:241
  11: enum$<core::result::Result<iron::response::Response, iron::error::IronError>>::or_else<iron::response::Response,iron::error::IronError,iron::error::IronError,closure-2>
             at /rustc/cc77ba46fcb2d288aa01554b48cd586c5827c3dd\library\core\src\result.rs:770
  12: docs_rs::web::{{impl}}::handle
             at src\web\mod.rs:183
  13: iron::iron::{{impl}}::handle<docs_rs::web::MainHandler>
             at C:\Users\Joshua Nelson\.cargo\registry\src\github.com-1ecc6299db9ec823\iron-0.6.1\src\iron.rs:176

thread '<unnamed>' panicked at 'error while serving error page: Error { kind: Msg("Failed to render 'error.html'"), source: Some(Error { kind: Msg("Variable `oops` not found in context while rendering 'error.html'"), source: None }) }', src\web\page\web_page.rs:72:20
```