warn, not error, on large return err variants

Author: BatmanAoD

crates/lib/src/lib.rs

@@ -4,6 +4,7 @@
 #![allow(clippy::missing_errors_doc)]
 #![allow(clippy::implicit_hasher)]
 #![allow(clippy::too_many_arguments)]
+#![warn(clippy::result_large_err)] // TODO #555: box the large variants or change the lint tthreshold
 #![forbid(unsafe_code)]
 #![warn(future_incompatible)]
 #![warn(rust_2018_compatibility, rust_2018_idioms)]