message: add custom error messages to secondary index get to match those of primary index get
diff: @@ -319,14 +319,14 @@ class multi_index
  return lb;
  }
  - const T& get( secondary_key_type&& secondary )const {
  - return get( secondary );
  + const T& get( secondary_key_type&& secondary, const char* error_msg = "unable to find secondary key" )const {
  + return get( secondary, error_msg );
  }
  // Gets the object with the smallest primary key in the case where the secondary key is not unique.
  - const T& get( const secondary_key_type& secondary )const {
  + const T& get( const secondary_key_type& secondary, const char* error_msg = "unable to find secondary key" )const {
  auto result = find( secondary );
  - eosio_assert( result != cend(), "unable to find secondary key" );
  + eosio_assert( result != cend(), error_msg );
  return *result;
  }
  