Replace ensure with if-statements (Prague)

[SamWilsn]

What was wrong?

Related to Issue #588

How was it fixed?

See #932

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 1.04167% with 95 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (forks/prague@afa50ad). Click here to learn what that means.

Files	Patch %	Lines
src/ethereum/prague/fork.py	0.00%	52 Missing ⚠️
src/ethereum/prague/vm/instructions/system.py	0.00%	8 Missing ⚠️
src/ethereum/prague/vm/instructions/storage.py	0.00%	6 Missing ⚠️
...rague/vm/precompiled_contracts/point_evaluation.py	0.00%	6 Missing ⚠️
src/ethereum/prague/vm/instructions/stack.py	0.00%	4 Missing ⚠️
src/ethereum/prague/vm/interpreter.py	0.00%	4 Missing ⚠️
...ereum/prague/vm/precompiled_contracts/alt_bn128.py	0.00%	4 Missing ⚠️
...thereum/prague/vm/precompiled_contracts/blake2f.py	0.00%	4 Missing ⚠️
src/ethereum/prague/state.py	0.00%	2 Missing ⚠️
src/ethereum/prague/vm/instructions/environment.py	0.00%	2 Missing ⚠️
... and 2 more

Additional details and impacted files

@@               Coverage Diff               @@
##             forks/prague     #944   +/-   ##
===============================================
  Coverage                ?   70.60%           
===============================================
  Files                   ?      684           
  Lines                   ?    37890           
  Branches                ?        0           
===============================================
  Hits                    ?    26751           
  Misses                  ?    11139           
  Partials                ?        0           

Flag	Coverage Δ
unittests	70.60% <1.04%> (?)

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