errors: expose pub getters for type-erased errors

Sometimes, user might want to downcast `Arc<dyn Error>` to specific
error type.

Actually, there is a use case for this in cpp-rust-driver,
where we need to define custom `SerializeRow` implementation. It returns
a custom error type. We would like to match against it during
rust-to-C error conversion - downcasting is thus required.

scylla-cql/src/types/deserialize/mod.rs

@@ -211,6 +211,11 @@ impl TypeCheckError {
     pub fn new(err: impl std::error::Error + Send + Sync + 'static) -> Self {
         Self(Arc::new(err))
     }
+
+    /// Retrieve an error reason.
+    pub fn get_reason(&self) -> &Arc<dyn std::error::Error + Send + Sync> {
+        &self.0
+    }
 }
 
 /// An error indicating that a failure happened during deserialization.
@@ -236,6 +241,11 @@ impl DeserializationError {
     pub fn new(err: impl Error + Send + Sync + 'static) -> Self {
         Self(Arc::new(err))
     }
+
+    /// Retrieve an error reason.
+    pub fn get_reason(&self) -> &Arc<dyn Error + Send + Sync> {
+        &self.0
+    }
 }
 
 impl Display for DeserializationError {

scylla-cql/src/types/serialize/mod.rs

@@ -41,6 +41,11 @@ impl SerializationError {
     pub fn new(err: impl Error + Send + Sync + 'static) -> SerializationError {
         SerializationError(Arc::new(err))
     }
+
+    /// Retrieve an error reason.
+    pub fn get_reason(&self) -> &Arc<dyn Error + Send + Sync> {
+        &self.0
+    }
 }
 
 impl Display for SerializationError {