Don't mask BIG (sync) establishment failure with a KeyError

[devitwise]

Fixes #954.

On a BIG or BIG-sync establishment failure, the event handler (on_big_establishment / on_big_sync_establishment) removes the bigs / big_syncs entry and emits ESTABLISHMENT_FAILURE, which makes the awaited future raise hci.HCI_Error. create_big() / create_big_sync() then ran del self.bigs[big_handle] / del self.big_syncs[big_handle] in their except hci.HCI_Error clause on an entry that was already gone, raising KeyError: 0 and discarding the real HCI status (e.g. a wrong broadcast code surfaces as KeyError: 0 instead of CONNECTION_TERMINATED_DUE_TO_MIC_FAILURE).

This pops with a default instead of deleting, matching the idempotent pattern already used in on_big_termination(). The HCI_Error now propagates with its status intact.

Test: tests/device_test.py::test_create_big_sync_failure_propagates_hci_error reproduces the failure sequence (handler removes the entry, then the except clause runs); it raises KeyError: 0 before the change and the correct HCI_Error after. Full device_test.py + hci_test.py suites green.

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[devitwise]

@googlebot I signed it!