[trivial] Change check invariant failure log level to fatal

[seelabs]

No description provided.

[codecov-io]

Codecov Report

Merging #2154 into develop will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #2154      +/-   ##
===========================================
- Coverage    69.01%   68.99%   -0.03%     
===========================================
  Files          684      684              
  Lines        50468    50468              
===========================================
- Hits         34833    34818      -15     
- Misses       15635    15650      +15

Impacted Files	Coverage Δ
src/ripple/app/tx/impl/ApplyContext.cpp	100% <100%> (ø)	⬆️
src/ripple/app/misc/detail/WorkBase.h	69.81% <0%> (-11.33%)	⬇️
src/ripple/basics/DecayingSample.h	77.77% <0%> (-8.34%)	⬇️
src/ripple/app/ledger/PendingSaves.h	93.54% <0%> (-6.46%)	⬇️
src/ripple/app/misc/impl/ValidatorSite.cpp	78.47% <0%> (-0.7%)	⬇️
src/ripple/server/impl/BaseWSPeer.h	67.5% <0%> (-0.63%)	⬇️
src/ripple/core/impl/JobQueue.cpp	86.32% <0%> (-0.48%)	⬇️
src/ripple/app/ledger/impl/LedgerMaster.cpp	44.04% <0%> (-0.23%)	⬇️
src/ripple/protocol/impl/Serializer.cpp	70.3% <0%> (+0.34%)	⬆️

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[ximinez]

👍

[nbougalis]

Merged as d831328