update retcode from symbol

[kaitlyn-loftus]

fix terminate! to update solution's retcode with a SciMLBase.ReturnCode instead of a Symbol (raises warning)

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Followed what's in OrdinaryDiffEq.jl

[devmotion]

Great, thank you! Is the warning a deprecation warning or shown unconditionally? In the latter case it might be nice to add a test that checks that it does not appear anymore with the updated code.

[kaitlyn-loftus]

I'm not sure @devmotion. The warning is from here.

[devmotion]

I added a simple test, thank you!