Fix misleading error message in Engine::export()

[coderabbitai]

Description

The error message in Engine::export() method says "Failed to get torrents", but the operation is actually exporting the engine state. This could confuse debugging efforts.

Location

File: crates/libtortillas/src/engine/mod.rs
Line: 276

The error message should be changed to accurately reflect the operation being performed (e.g., "Failed to export engine state").

References

• PR: feat: Exportable engine state #166
• Comment: feat: Exportable engine state #166 (comment)
• Requested by: @artrixdotdev