Fix watching when paths don't exist

[simon-katz]

This fixes a problem where watching silently fails to happen when paths don't exist.

Tests (with bin/kaocha) are still passing.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (d390b8e) 75.08% compared to head (6c45d0b) 75.09%. Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #425 +/- ## ======================================= Coverage 75.08% 75.09% ======================================= Files 52 52 Lines 2802 2803 +1 Branches 289 289 ======================================= + Hits 2104 2105 +1 Misses 511 511 Partials 187 187 ``` | [Flag](https://app.codecov.io/gh/lambdaisland/kaocha/pull/425/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/lambdaisland/kaocha/pull/425/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `56.29% <0.00%> (-0.03%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/lambdaisland/kaocha/pull/425/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `69.42% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/lambdaisland/kaocha/pull/425?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/kaocha/watch.clj](https://app.codecov.io/gh/lambdaisland/kaocha/pull/425?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2thb2NoYS93YXRjaC5jbGo=) | `71.67% <100.00%> (+0.12%)` | :arrow_up: |

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[plexus]

Released in v1.86.1355

[lambdaisland/kaocha "1.86.1355"]                 ;; deps.edn
{lambdaisland/kaocha {:mvn/version "1.86.1355"}}  ;; project.clj