EPUB.Writer.convert! returns error tuples instead of raising

[StephanMeijer]

Problem

DocSpec.Core.EPUB.Writer.convert!/1 and convert!/2 violate Elixir's bang function convention. Bang functions should raise on error and return bare values, but these return {:ok, binary()} | {:error, term()}:

@spec convert!(spec :: DocumentSpecification.t()) :: {:ok, binary()} | {:error, term()}

This surprises every Elixir developer who uses the library — calling convert! and getting back an ok/error tuple is a contract violation.

Expected behavior

Following Elixir convention:

• convert/1 → {:ok, binary()} | {:error, term()}
• convert!/1 → binary() (raises on error)

Files

• lib/docspec/core/epub/writer.ex
• lib/docspec/cli.ex (caller)

Notes

This is a breaking change for the public API. Recommend shipping with a major or minor version bump.