[ntuple] Throw exception if appended file is not writable

[silverweed]

instead of crashing.

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    22 files      22 suites   3d 20h 18m 24s ⏱️
 3 814 tests  3 813 ✅ 0 💤 1 ❌
76 772 runs  76 771 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 91b357b.