Fix: average_mmr may be optional

src/error.rs

@@ -6,10 +6,10 @@ pub enum Error {
     ZipError(#[from] zip::result::ZipError),
 
     #[error("Protocol Buffers error")]
-    DecodeFailed(#[from] prost::DecodeError),
+    ProtobufError(#[from] prost::DecodeError),
 
     #[error("Pickle error")]
-    UnpickleFailed(#[from] serde_pickle::Error),
+    PickleError(#[from] serde_pickle::Error),
 
     #[error("I/O error")]
     IoError(#[from] std::io::Error),
@@ -19,7 +19,7 @@ pub enum Error {
 
     #[cfg(feature = "meta")]
     #[error("JSON error")]
-    JsonDecodeFailed(#[from] serde_json::Error),
+    JsonError(#[from] serde_json::Error),
 
     #[error("invalid magic: {0:#X}, expected: {1:#X}")]
     InvalidMagic(u32, u32),