fix: consistent error handling + exit codes in wallet balance check CLI [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Changes

Fixes two error-handling inconsistencies in tools/rustchain_wallet_cli.py (bounty rustchain-bounties#16253).

1. cmd_export — explicit error handling for missing wallet

Previously, cmd_export let _load_keystore's FileNotFoundError bubble up to main()'s catch-all, which returned the generic EXIT_UNKNOWN_ERROR (6). Now it returns EXIT_WALLET_NOT_FOUND (4) with a clear error message, matching the pattern used by cmd_send.

2. main() catch-all — consistent error message casing

Changed "Error: {e}" (lowercase) to "ERROR: {e}" (uppercase) to match the consistent uppercase style used by every other error path (cmd_balance, cmd_send, cmd_history, cmd_miners, cmd_epoch).

3. Comprehensive test coverage (15 new tests)

Added tests/test_wallet_cli_error_handling_16253.py covering every failure path:

Command	Path	Expected Exit Code
_safe_json	Non-JSON body (502)	EXIT_BAD_RESPONSE (3)
_safe_json_object	JSON array where object expected	EXIT_BAD_RESPONSE (3)
cmd_balance	Network error	EXIT_NETWORK_ERROR (2)
cmd_balance	404 not found	EXIT_WALLET_NOT_FOUND (4)
cmd_balance	500 server error	EXIT_BAD_RESPONSE (3)
cmd_balance	Malformed JSON body	EXIT_BAD_RESPONSE (3)
cmd_balance	Missing amount field	EXIT_BAD_RESPONSE (3)
cmd_balance	balance_rtc alias (happy path)	EXIT_SUCCESS (0)
cmd_export	Missing wallet	EXIT_WALLET_NOT_FOUND (4)
cmd_history	Network error	EXIT_NETWORK_ERROR (2)
cmd_history	404 not found	EXIT_WALLET_NOT_FOUND (4)
cmd_history	500 server error	EXIT_BAD_RESPONSE (3)
cmd_send	Missing wallet	EXIT_WALLET_NOT_FOUND (4)
cmd_send	Network error	EXIT_NETWORK_ERROR (2)

Specific inconsistencies addressed (from #7889)

• cmd_export: FileNotFoundError bubbled to main() catch-all → returned wrong exit code (6 instead of 4)
• main(): lowercase "Error: {e}" inconsistent with uppercase "ERROR: " everywhere else
• All failure paths now write to stderr with clear error message and appropriate non-zero exit code
• Exit code scheme already documented in --help epilog

Verification

• python3 -m pytest tests/test_wallet_cli_39.py tests/test_wallet_cli_error_handling_16253.py --noconftest -v → 31/31 passed
• No behavior change on the happy path

Closes #7889
Closes Scottcjn/rustchain-bounties#16253

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