Fix error naming conventions

[d3v-active]

Closes #502

This PR reviews and standardizes the error naming conventions and documentation across all smart contracts in the repository to ensure they follow NatSpec (Rust documentation) standards.

Changes Made

While the error enums were already correctly named using standard Rust/Soroban conventions (e.g., SplitError, TokenError, etc.), many of the variants lacked descriptive documentation, making it harder for developers and users to understand why an operation failed.

The following contracts were updated to include clear and accurate docstrings for all custom error variants:

• split/src/lib.rs: Added complete documentation for SplitError.
• token/src/lib.rs: Added complete documentation for TokenError.
• vesting/src/lib.rs: Added complete documentation for VestingError.
• wrapper/src/lib.rs: Added complete documentation for all previously undocumented variants of WrapperError.

Acceptance Criteria Met

• Documentation is clear and accurate.
• Linter/Formatter passes with 0 warnings (cargo fmt and cargo clippy were run to verify).
• Follows standard developer specification comments (equivalent to NatSpec in Solidity).

Impact

No business logic was changed. These are purely documentation enhancements to improve the developer experience and maintain code quality and consistency across the project.

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