blockstorage: allow reading partial block data from storage

It will allow fetching specific transactions using an external index,
following https://github.com/bitcoin/bitcoin/pull/32541#issuecomment-3267485313.

No logging takes place in case of an invalid offset/size (to avoid spamming the log),
by using a new `ReadRawError::BadPartRange` error variant.

Co-authored-by: Hodlinator <172445034+hodlinator@users.noreply.github.com>
Co-authored-by: Lőrinc <pap.lorinc@gmail.com>

src/rest.cpp

@@ -420,6 +420,7 @@ static bool rest_block(const std::any& context,
     if (!block_data) {
         switch (block_data.error()) {
         case node::ReadRawError::IO: return RESTERR(req, HTTP_INTERNAL_SERVER_ERROR, "I/O error reading " + hashStr);
+        case node::ReadRawError::BadPartRange: break; // can happen only when reading a block part
         }
         assert(false);
     }