Change RangeProofError to RangeTooSmall

This is the only kind of RangeProofError
From review comments

firewood/src/db.rs

@@ -351,7 +351,7 @@ impl<S: ShaleStore<Node> + Send + Sync> api::DbView for DbRev<S> {
 
         // remove the last key from middle and do a proof on it
         let last_key = match middle.pop() {
-            None => return Err(api::Error::RangeProofError("Range too small".to_owned())),
+            None => return Err(api::Error::RangeTooSmall),
             Some((last_key, _)) => self.single_key_proof(last_key).await,
         };
 

firewood/src/v2/api.rs

@@ -73,8 +73,8 @@ pub enum Error {
     #[error("Internal error")]
     InternalError(Box<dyn std::error::Error + Send>),
 
-    #[error("RangeProofError")]
-    RangeProofError(String),
+    #[error("Range too small")]
+    RangeTooSmall,
 }
 
 /// A range proof, consisting of a proof of the first key and the last key,