EC2: refactor DryRun exception

[bpandola]

• Move DryRunOperation exception and helper method from core to ec2 backend.
• Add test helper to encapsulate DryRun error assertions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.02%. Comparing base (5717997) to head (f124ba2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9449   +/-   ##
=======================================
  Coverage   93.02%   93.02%           
=======================================
  Files        1293     1293           
  Lines      116229   116225    -4     
=======================================
+ Hits       108122   108123    +1     
+ Misses       8107     8102    -5     

Flag	Coverage Δ
servertests	28.83% <92.30%> (+<0.01%)	⬆️
unittests	93.00% <100.00%> (+<0.01%)	⬆️

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