Add "too large" error message, and explain file limit and transcoding in the help text

[audiodude]

Fixes #21

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e4dc4fe) 95.07% compared to head (37ef17c) 95.07%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #24 +/- ## ======================================= Coverage 95.07% 95.07% ======================================= Files 24 24 Lines 1218 1218 ======================================= Hits 1158 1158 Misses 60 60 ```

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