Catch catastrophic solver failure when building MIS

[bknueven]

Fixes IDAES/idaes-pse#1520

Summary/Motivation:

The minimal intractable system calculation had a try/except around a solve we expect to fail, but when we check for failure we failed to first check the results object against None.

Changes proposed in this PR:

• Insert results is not None conditional to short-circuit, thus not calling pyo.check_optimal_termination(results) if results is None.

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[jsiirola]

Question: would it better to not swallow all exceptions from the solve() above and instead use load_solutions=False in the call? Then results should never be None?

[bknueven]

Probably. Would that still not raise an exception if the solver returns a non-zero exit code? I think that's why I've implemented these as blanket try / except blocks.

[jsiirola]

OK. I would just be worried about the bare except: -- that could be suppressing any number of errors and there would be no way to know what was going on.

[jsiirola]

I am OK merging this as it resolves a serious error blocking MIS users. However, this highlights fragility in the MIS implementation due to the use of a bare (catch-all) "except:" clause.

It would be good to revisit this and see if the bare "except:" can be removed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.66%. Comparing base (c463632) to head (57f5e06).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3403      +/-   ##
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+ Coverage   88.63%   88.66%   +0.03%     
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  Files         879      879              
  Lines      100363   100363              
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+ Hits        88955    88986      +31     
+ Misses      11408    11377      -31     

Flag	Coverage Δ
linux	86.12% <100.00%> (-0.01%)	⬇️
osx	76.11% <0.00%> (ø)
other	86.61% <100.00%> (-0.01%)	⬇️
win	84.46% <100.00%> (ø)

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