refactor(tvix/derivation): rename {ValidateDerivation,}Error

This is now used in more than just validate().

Change-Id: I69c3ad6cb5f3ad60a636fe2ea05d432aebe8e53b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7851
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/derivation/src/errors.rs

@@ -3,7 +3,7 @@ use tvix_store::store_path::ParseStorePathError;
 
 /// Errors that can occur during the validation of Derivation structs.
 #[derive(Debug, Error)]
-pub enum ValidateDerivationError {
+pub enum DerivationError {
     // outputs
     #[error("No outputs defined.")]
     NoOutputs(),