return more specific error message when attemting to decode UDF volume

[codecov[bot]]

Codecov Report

Patch coverage: 73.33% and project coverage change: +0.17 :tada:

Comparison is base (1cdbb13) 75.54% compared to head (9ea3efa) 75.72%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #30 +/- ## ========================================== + Coverage 75.54% 75.72% +0.17% ========================================== Files 4 4 Lines 957 964 +7 ========================================== + Hits 723 730 +7 Misses 165 165 Partials 69 69 ``` | [Impacted Files](https://codecov.io/gh/kdomanski/iso9660/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski) | Coverage Δ | | |---|---|---| | [image\_writer.go](https://codecov.io/gh/kdomanski/iso9660/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski#diff-aW1hZ2Vfd3JpdGVyLmdv) | `75.98% <55.55%> (+0.21%)` | :arrow_up: | | [iso9660.go](https://codecov.io/gh/kdomanski/iso9660/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kamil+Doma%C5%84ski#diff-aXNvOTY2MC5nbw==) | `73.15% <100.00%> (+0.22%)` | :arrow_up: |

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