Throw type_error.321 for discarded values in to_bon8

write_bon8_value silently skipped discarded values, but the array/object
count marker still counted them, so [1, discarded] produced 82 91: a marker
announcing two elements followed by one. Throw type_error.321 like the other
binary writers do, at any nesting level.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/output/binary_writer.hpp

@@ -809,6 +809,8 @@ class binary_writer
 
     /*!
     @param[in] j  JSON value to serialize
+
+    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_bon8(const BasicJsonType& j)
     {
@@ -2570,6 +2572,8 @@ class binary_writer
     @param[in] j                JSON value to serialize
     @param[in,out] string_open  whether the output ends with a non-empty
                                 string that has not been terminated with 0xFF
+
+    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_bon8_value(const BasicJsonType& j, bool& string_open)
     {
@@ -2678,7 +2682,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, "BON8");
         }
     }