fw: gracefully handle case when no releases were rolled out yet

[mniestroj]

So far those log messages were printed when there were no dfu releases rolled out yet:

  <err> golioth: Failed to get manifest bstr: 34 (QCBOR_ERR_LABEL_NOT_FOUND)
  <err> golioth_dfu: Failed to parse desired version: -13

Handle the case when sequence-number is not found in CBOR map and return -ENOENT to signal no "desired version". Handle that in samples/dfu/ application, so that following log messages are printed instead:

  <inf> golioth_dfu: No release rolled out yet

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 83338b8):

https://golioth-zephyr-sdk-doxygen-dev--pr284-fw-handle-no-rol-qi5d2crh.web.app

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