Fix QPU failover - signal solver failover condition (on resolve)

[randomir]

Change in DWaveSampler/DWaveCliqueSampler behavior: when failover=True is specified on construction, solver failover is only signalled, and the actual failover is left as an exercise for the developer. 😆

The "dirty" bit we solve here is SAPI/solver exception unification:

• if we think sampling call is worth retrying on the same solver, we'll raise RetryCondition on sampleset resolve; and
• if we think sampling should be retried on a different solver, we'll raise FailoverCondition on sampleset resolve.

Note that RetryCondition is a specialization of FailoverCondition, so catching FailoverCondition and doing sampler.trigger_failover() should handle all failover cases.

[codecov-commenter]

Codecov Report

Merging #465 (3f75aad) into master (6a27508) will decrease coverage by 4.77%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
- Coverage   91.13%   86.36%   -4.78%     
==========================================
  Files          23       23              
  Lines        1546     1540       -6     
==========================================
- Hits         1409     1330      -79     
- Misses        137      210      +73     

Impacted Files	Coverage Δ
dwave/system/samplers/dwave_sampler.py	82.03% <91.66%> (-4.02%)	⬇️
dwave/system/exceptions.py	100.00% <100.00%> (ø)
dwave/system/samplers/clique.py	73.85% <100.00%> (-13.97%)	⬇️
dwave/system/coupling_groups.py	12.50% <0.00%> (-81.25%)	⬇️
dwave/system/samplers/leap_hybrid_sampler.py	61.42% <0.00%> (-14.29%)	⬇️
dwave/system/composites/embedding.py	96.57% <0.00%> (-1.15%)	⬇️
dwave/system/testing.py	95.12% <0.00%> (-0.82%)	⬇️

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[arcondello]

LGTM!

[arcondello]

Looks great!