Add error handling

[rameloni]

Some functions in the library do not implement error handling. It is better to return Result instead of panicking. Thus, any function that may fail should return Result.

[rameloni]

• hgldd::reader https://github.com/rameloni/tywaves-rs/blob/main/src/hgldd/reader.rs
• tyvcd::builder https://github.com/rameloni/tywaves-rs/blob/main/src/tyvcd/builder.rs
• vcd_rewrite https://github.com/rameloni/tywaves-rs/blob/main/src/vcd_rewrite.rs