editoast: make 'Result' more flexible

When defining a new `Result`, it's sometimes annoying having a
`Result` available and trying to use it as `Result<T, E>` but actually,
there is only one generic argument.

Replacing `type Result<T> = StdResult<T, InternalError>` by
`type Result<T, E = InternalError> = StdResult<T, E>` keep the
bonus of using a custom `Result` (always returning our `InternalError`)
but leaving the possibility to opt-out easily by specifying the second
generic argument.

editoast/src/error.rs

@@ -19,7 +19,7 @@ crate::schemas! {
     InternalError,
 }
 
-pub type Result<T> = StdResult<T, InternalError>;
+pub type Result<T, E = InternalError> = StdResult<T, E>;
 
 /// Trait for all errors that can be returned by editoast
 pub trait EditoastError: Error + Send + Sync {