Tidy up some metadata accessing (#978)

* Reduce some repetition when obtaining metadata pallets/runtime_traits

* make them pub

* fix docs and clippy

subxt/src/metadata/metadata_type.rs

@@ -2,6 +2,7 @@
 // This file is dual-licensed as Apache-2.0 or GPL-3.0.
 // see LICENSE for license details.
 
+use crate::error::MetadataError;
 use std::sync::Arc;
 
 /// A cheaply clone-able representation of the runtime metadata received from a node.
@@ -23,6 +24,33 @@ impl Metadata {
             inner: Arc::new(md),
         }
     }
+
+    /// Identical to `metadata.pallet_by_name()`, but returns an error if the pallet is not found.
+    pub fn pallet_by_name_err(
+        &self,
+        name: &str,
+    ) -> Result<subxt_metadata::PalletMetadata, MetadataError> {
+        self.pallet_by_name(name)
+            .ok_or_else(|| MetadataError::PalletNameNotFound(name.to_owned()))
+    }
+
+    /// Identical to `metadata.pallet_by_index()`, but returns an error if the pallet is not found.
+    pub fn pallet_by_index_err(
+        &self,
+        index: u8,
+    ) -> Result<subxt_metadata::PalletMetadata, MetadataError> {
+        self.pallet_by_index(index)
+            .ok_or(MetadataError::PalletIndexNotFound(index))
+    }
+
+    /// Identical to `metadata.runtime_api_trait_by_name()`, but returns an error if the trait is not found.
+    pub fn runtime_api_trait_by_name_err(
+        &self,
+        name: &str,
+    ) -> Result<subxt_metadata::RuntimeApiMetadata, MetadataError> {
+        self.runtime_api_trait_by_name(name)
+            .ok_or_else(|| MetadataError::RuntimeTraitNotFound(name.to_owned()))
+    }
 }
 
 impl From<subxt_metadata::Metadata> for Metadata {