[ERROR] Make Error conform more to std

[tqchen]

This PR changes Error.what() to only return message. This helps to conform to the standard that what should not throw. Added a error.FullMessage() function to get the fullmessage with trace and kind.

This also helps us to remove TLS in the header side which can makes the compiled c++ object more friendly wrt to LLVM JIT.

[gemini-code-assist]

Summary of Changes

Hello @tqchen, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the Error class to improve its adherence to the std::exception interface and enhance compatibility with LLVM JIT. It achieves this by separating the concise error message (now returned by what()) from the comprehensive error details (now available via a new FullMessage() method), while also removing Thread-Local Storage from the header.

Highlights

• Standard Conformance: The Error::what() method now strictly returns only the error message, aligning with std::exception's contract that what() should not throw.
• New Full Message Functionality: A new method, Error::FullMessage(), has been introduced to provide the complete error details, including the traceback, error kind, and message.
• JIT Compatibility Improvement: This refactoring eliminates the need for Thread-Local Storage (TLS) in the error header, which enhances compatibility and friendliness for LLVM JIT compilation.

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[gemini-code-assist]

Code Review

This pull request refactors the Error class to better align with the C++ standard by making what() noexcept and only return the error message. The full error message, including traceback, is now available through a new FullMessage() method. This is a good change that improves standard compliance and correctness. The tests have been updated accordingly. I've suggested a small improvement to the implementation of FullMessage() for better efficiency and readability.