db: catch BDB exceptions so unreadable files fail cleanly (#34) (#38)

Berkeley DB throws when it cannot open a file written by
another platform's build (e.g. BDB 5.3 wallet read by 6.2).
Nothing on the call path caught these, so the exception
unwound into std::terminate -- surfacing on Windows as
STATUS_STACK_BUFFER_OVERRUN (0xC0000409) with no useful
message.

Three layers of defense:

1. main_gui.cpp: try/catch around LoadBlockIndex() and
   LoadWallet(), printing the reason and exiting cleanly.

2. db.cpp LoadWallet: try/catch around the record-reading
   loop (Satoshi's 2009 TODO), tracking the last record type
   so the error message says what it choked on.

3. db.cpp cleanup paths: every dbenv.close(), pdb->close(),
   txn_checkpoint() and log_archive() wrapped in try/catch
   so the environment teardown after a failed open does not
   throw a second exception into std::terminate.

Tested: cross-platform wallet (Linux BDB 5.3 → Windows 6.2)
now prints a clear message and exits with code 1, wallet
left untouched.

Closes #34

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

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