Mimic camchem substep convergence failure integration acceptance

[K20shores]

If the Backward Euler solver fails to converge, the state at the end of the last successful solver time step should be returned.

Acceptance Criteria

• When the Backward Euler solver fails to converge, the state at the end of the last successful solver time step is returned.

Ideas

• The Robertson analytical example can be used to test this. As of now, backward euler fails to converge for the robertson example
• This block of code could be modified to set state.variables_ = Yn prior to throwing then error: https://github.com/NCAR/micm/blob/b819e5770cd7e83d60d23181c2a90e1bbd660afb/include/micm/solver/backward_euler.inl#L171-L179