improve dtmin and unstable detection

[oscardssmith]

Followup to #692. It turns out that OrdinaryDiffEq sets the minimum dt to eps(t) which is relatively sensible, but that means that we should be failing with an error if we don't accept a step and we're already at the minimum dt.

This definitely needs some tests, but I think those tests need to be in OrdinaryDiffEq so they will be added as a separate PR once this merges.

[codecov]

Codecov Report

Attention: Patch coverage is 32.00000% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 29.31%. Comparing base (2df59ce) to head (761a8c7).
Report is 4 commits behind head on master.

Files	Patch %	Lines
src/integrator_interface.jl	32.00%	17 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #693       +/-   ##
===========================================
- Coverage   41.40%   29.31%   -12.10%     
===========================================
  Files          55       55               
  Lines        4557     4551        -6     
===========================================
- Hits         1887     1334      -553     
- Misses       2670     3217      +547     

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[oscardssmith]

Re-running CI (now that it's morning so codecov will hopefully be happier and now that the OrdinaryDiffEq test that I think was the only real failure is fixed).