enable error_impl_error lint

Except the 1 violation would have to be renamed in, like, every single
file in this project. So we're just enabling it so that we don't make
the same mistake in the future.

src/utils/error.rs

@@ -16,6 +16,7 @@ use crate::RumaResponse;
 pub(crate) type Result<T, E = Error> = std::result::Result<T, E>;
 
 #[derive(Error, Debug)]
+#[allow(clippy::error_impl_error)]
 pub(crate) enum Error {
     #[cfg(feature = "sqlite")]
     #[error("There was a problem with the connection to the sqlite database: {source}")]