Fix misleading error messages in conversions dispatch fallbacks

[takayoshi-makabe]

Rationale for this change

The singledispatch fallbacks in conversions.py describe the wrong operation, so an unregistered type produces a message that sends you looking in the wrong place. Passing a type with no registered implementation to each of the five entry points today:

partition_to_py  Cannot convert 'foo' to unsupported type: FooUnknownType()
to_bytes         scale does not match FooUnknownType()
from_bytes       Cannot deserialize bytes, type FooUnknownType() not supported: b'foo'
to_json          Cannot deserialize bytes, type FooUnknownType() not supported: foo
from_json        Cannot deserialize bytes, type FooUnknownType() not supported: foo

This reworks the three to follow the shape from_bytes already uses, so each names the operation it performs and the value it rejected:

to_bytes         Cannot serialize to bytes, type FooUnknownType() not supported: b'foo'
to_json          Cannot serialize to JSON, type FooUnknownType() not supported: foo
from_json        Cannot deserialize JSON, type FooUnknownType() not supported: foo

partition_to_py and from_bytes are already accurate and are left alone.

One small change beyond the strings: to_bytes takes its value parameter as value instead of _ so the message can include it, matching the other four. It is rendered with !r, like from_bytes, because the parameter also accepts bytes.

Are these changes tested?

Yes. test_raise_on_unregistered_type asserted the old to_bytes wording, so it is updated. to_json and from_json had no fallback coverage at all — which is how the copied text survived — so cases for both are added and all five entry points are now covered.

Are there any user-facing changes?

Only the text of these TypeError messages. No behaviour changes: the same inputs raise the same exception type in the same places.