simplify retrieval in find_by_host()

we can use "first()" instead of limit(1).load().etc…
since on a UNIQUE field, we only expect 1 result.

first() returns QueryResult, which is Result<T, diesel::Error>, so we
need to implement a converter for that error type.

This commit addresses @fdb-hiroshima's review.

src/routes/errors.rs

@@ -14,6 +14,12 @@ impl From<Error> for ErrorPage {
     }
 }
 
+impl From<diesel::result::Error> for ErrorPage {
+    fn from(err: diesel::result::Error) -> ErrorPage {
+        ErrorPage(plume_models::Error::Db(err))
+    }
+}
+
 impl<'r> Responder<'r> for ErrorPage {
     fn respond_to(self, req: &Request) -> response::Result<'r> {
         let rockets = req.guard::<PlumeRocket>().unwrap();