Handle read_dir() errors in persister

[andrei-21]

No description provided.

[TheBlueMatt]

This LGTM (great catch!), but I genuinely do not understand why CI had failed. It had this output on a single job but I kicked it to see if its reproducible.

 ---- tests::test_if_monitors_is_not_dir stdout ----
Using Block Connection Style: BestBlockFirstReorgsOnlyTip
thread 'tests::test_if_monitors_is_not_dir' panicked at 'assertion failed: persister.read_channelmonitors(nodes[0].keys_manager).is_err()', lightning-persister/src/lib.rs:180:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Failed to remove test persister directory: No such file or directory (os error 2)


failures:
    tests::test_if_monitors_is_not_dir

[codecov-commenter]

Codecov Report

Base: 90.75% // Head: 90.72% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (7afa230) compared to base (197a47a).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1943      +/-   ##
==========================================
- Coverage   90.75%   90.72%   -0.03%     
==========================================
  Files          96       96              
  Lines       50135    50149      +14     
  Branches    50135    50149      +14     
==========================================
- Hits        45498    45496       -2     
- Misses       4637     4653      +16     

Impacted Files	Coverage Δ
lightning-persister/src/lib.rs	93.49% <100.00%> (+0.83%)	⬆️
lightning/src/chain/mod.rs	63.63% <0.00%> (-4.55%)	⬇️
lightning/src/chain/onchaintx.rs	94.33% <0.00%> (-1.05%)	⬇️
lightning-net-tokio/src/lib.rs	77.01% <0.00%> (-0.30%)	⬇️
lightning/src/ln/functional_tests.rs	96.97% <0.00%> (-0.14%)	⬇️
lightning/src/chain/channelmonitor.rs	91.03% <0.00%> (-0.05%)	⬇️

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[andrei-21]

The problem was that the test relied on a preexistence directory, but removed it at the end. So subsequent runs were failing because there were no expected directory anymore.
I fixed it by creating the required directory (and the file) at the beginning of the test.