P2P: Reduce duplicate trx debug log output

[heifner]

See Wire-Network/wire-sysio#171

Previous:

debug 2026-01-02T16:17:42.037 nodeop    net_plugin.cpp:4044           operator()           ] bad packed_transaction : 3040008 tx_duplicate: Duplicate transaction
duplicate transaction 24980a3bd2035890209ac387ef899b217b82f9c2c8c0b9167f3836e9a89159da
    {"id":"24980a3bd2035890209ac387ef899b217b82f9c2c8c0b9167f3836e9a89159da"}
    nodeop  producer_plugin.cpp:1104 process_incoming_transaction_async

With PR:

debug 2026-01-12T18:41:06.076 nodeop    net_plugin.cpp:4031           operator()           ] bad packed_transaction : Duplicate transaction (3040008) duplicate transaction ea986755390b8fe89ca53c691d68b88869775f400b33c41c0a1dadbb1ae0a435

Includes some exception implementation cleanup.

• exception::to_string() now returns error string without newlines.