Send '503 Service Unavailable' when handling a request already timed out

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`IllegalStateException: Request already timed out` has to be translated to `503 Service Unavailable` but it is currently being translated to `500 Internal Server Error`:

```
[WARN] (CentralDogmaExceptions.java:77) Unexpected exception with error code INTERNAL_SERVER_ERROR (2) from: watchFile()
com.linecorp.centraldogma.internal.thrift.CentralDogmaException: java.lang.IllegalStateException: Request already timed out.
at com.linecorp.centraldogma.server.internal.thrift.Converter.convert(Converter.java:215)
at com.linecorp.centraldogma.server.internal.thrift.CentralDogmaServiceImpl.logAndInvokeOnError(CentralDogmaServiceImpl.java:396)
at com.linecorp.centraldogma.server.internal.thrift.CentralDogmaServiceImpl.lambda$handleWatchFileResult$24(CentralDogmaServiceImpl.java:388)
at java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:822)
at java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:797)
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:474)
at java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:1977)
at com.linecorp.armeria.common.util.Functions.lambda$voidFunction$0(Functions.java:182)
at java.util.concurrent.CompletableFuture.uniExceptionally(CompletableFuture.java:870)
at java.util.concurrent.CompletableFuture$UniExceptionally.tryFire(CompletableFuture.java:852)
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:474)
at java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:1977)
at com.linecorp.centraldogma.server.internal.storage.repository.cache.CachingRepository.lambda$null$3(CachingRepository.java:199)
...
Caused by: java.lang.IllegalStateException: Request already timed out.
```

Start with Converter.convert at Converter.java:215 and trace how the IllegalStateException from CentralDogmaServiceImpl is mapped. Reproduce the request-timeout path shown in the stack trace, then add or update coverage for the expected 503 Service Unavailable response and verify that other unexpected exceptions remain 500.

Written by the indexing model from the issue text.