eternal-load: Propagate aio future exceptions as NProto::TError (on async read/write)

[dvrazumov]

For now there is no error info in logs if async operation fails in eternal-load. This PR propagates exception message up to TTestExecutor

Before:

  cloud/blockstore/tools/testing/eternal_tests/eternal-load/lib/test_executor.cpp:442
  RunCompletion(): requirement future.HasValue() failed

After:

2025-12-04T12:29:29.162259Z :ETERNAL_EXECUTOR ERROR: cloud/blockstore/tools/testing/eternal_tests/eternal-load/lib/test_executor.cpp:193: Can't write to file: (yexception) (No space left on device) async IO operation failed

[github-actions]

Note

This is an automated comment that will be appended during run.

🔴 linux-x86_64-relwithdebinfo: some tests FAILED for commit 62c0b8a.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
9636	9632	0	3	0	1	0

🔴 linux-x86_64-relwithdebinfo: some tests FAILED for commit 62c0b8a.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
49	46	0	3	0	0	0

🔴 linux-x86_64-relwithdebinfo: some tests FAILED for commit 62c0b8a.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
49	46	0	3	0	0	0

[github-actions]

Note

This is an automated comment that will be appended during run.

🔴 linux-x86_64-relwithdebinfo: some tests FAILED for commit bc42ae5.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
9625	9618	0	5	0	1	1

🟢 linux-x86_64-relwithdebinfo: all tests PASSED for commit bc42ae5.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
213	213	0	0	0	0	0