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>

docs/mkdocs/docs/api/basic_json/to_bon8.md

@@ -41,6 +41,8 @@ With (2), the bytes written before the exception remain in the output adapter.
   above 9223372036854775807, which BON8 cannot represent
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if `j` contains a string that is not
   valid UTF-8
+- Throws [type_error.321](../../home/exceptions.md#jsonexceptiontype_error321) if `j` or a value nested in it is
+  discarded; example: `"cannot serialize discarded value to BON8"`
 
 ## Complexity
 

docs/mkdocs/docs/home/exceptions.md

@@ -810,7 +810,7 @@ does not list an enumerator and it is therefore converted like the first listed
 A discarded value (one created by [`parse()`](../api/basic_json/parse.md) with a callback that returns `false` for the
 value, or by default-constructing a [`basic_json`](../api/basic_json/index.md) with
 [`value_t::discarded`](../api/basic_json/value_t.md)) was passed to a binary serialization function, either directly or
-nested in an array or object. There is no way to represent a discarded value in CBOR, MessagePack, UBJSON, BJData, or BSON.
+nested in an array or object. There is no way to represent a discarded value in CBOR, MessagePack, UBJSON, BJData, BSON, or BON8.
 
 !!! failure "Example message"