Ported (exceptions.jl) from Stingray (Python) to Julia

[JawadAhmadCS]

Changes Made:

• Added exceptions.jl in src/ directory.
• Implemented Base.showerror for better error message formatting.
• Updated test/runtests.jl to include exception handling test cases.
• Ensured all tests pass successfully.

✅ Test Results:

This contribution is part of my efforts for Google Summer of Code (GSoC) 2025 to work on Spectral Timing in Julia. Contributing to Stingray.jl helps me gain codebase familiarity and deepen my understanding for meaningful GSoC contributions.

[JawadAhmadCS]

Hey @matteobachetti , I have ported the exception handling from Stingray (Python) to Julia. I added Exceptions.jl, defined StingrayError, and implemented Base.showerror. All tests are passing successfully.

Looking forward to your review and feedback!

Thanks!

[JawadAhmadCS]

Everything looks good on my end, and all tests are passing. Please review and let me know if there's anything that needs to be changed or improved before merging. Thanks!