Enable building any ServerFnError<T> using ServerFnError::<T>::new

Previously it was only possible to build the `NoCustomError` case this
way.

server_fn/src/error.rs

@@ -181,7 +181,7 @@ pub enum ServerFnError<E = NoCustomError> {
     MissingArg(String),
 }
 
-impl ServerFnError<NoCustomError> {
+impl<T> ServerFnError<T> {
     /// Constructs a new [`ServerFnError::ServerError`] from some other type.
     pub fn new(msg: impl ToString) -> Self {
         Self::ServerError(msg.to_string())