**Problem**
When the remote cache server (e.g. bazel-remote using S3 for storage) reports an AC (Action Cache)
hit but the underlying CAS (Content Addressable Storage) blob is missing or
corrupt, ActionCache.cache propagates the resulting exception (typically
java.io.FileNotFoundException) directly to the SBT task engine process with no interception of the propogated error.
This causes a build failure instead of a graceful cache miss.

The three unguarded call sites are:
1. organicTask - syncBlobs after a successful put only caught NoSuchFileException,
   missing FileNotFoundException and other IO errors.
2. getWithFailure / readFromSymlink fast-path - syncBlobs inside flatMap with no
   exception handling.
3. getWithFailure main branch - both syncBlobs calls and the subsequent IO.read
   were completely unguarded.

**Solution**
Guard all three call sites with NonFatal catches:
- Cache read failures (getWithFailure) return Left(None) which the caller
  interprets as a cache miss, triggering organic recomputation.
- Cache write failures (organicTask) are demoted to a debug-level log; the task
  result that was already computed is returned successfully.

Two regression tests are added to ActionCacheTest:
1. Tests the main getWithFailure branch using the default relative-path converter.
2. Tests the readFromSymlink fast-path using an absolute-path converter so the
   symlink created on the first run is found by Files.isSymbolicLink on the second.