EC2: Raise when deleting VPCs with dependent Subnets

[viren-nadkarni]

AWS does not allow deleting VPCs that have subnets associated with it. This PR adds this behaviour to Moto.

[viren-nadkarni]

Moto Cfn evaluates deletion in sequence, so now, attempting to delete VPCs before subnets raises.

It's not clear to me where/if Moto figures out the right order of resource manipulation, or how much additional work is required.

This is a quick workaround to make the test pass in the interim.

[bblommers]

This is arguably a bug in Moto - we figure out the right order when updating CF resources, but not yet when deleting them. I'll raise a separate PR to fix this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.82%. Comparing base (b29a9c8) to head (4f2d4f0).
Report is 29 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8811      +/-   ##
==========================================
+ Coverage   92.80%   92.82%   +0.01%     
==========================================
  Files        1261     1266       +5     
  Lines      109888   110464     +576     
==========================================
+ Hits       101986   102536     +550     
- Misses       7902     7928      +26     

Flag	Coverage Δ
servertests	28.11% <0.00%> (-0.11%)	⬇️
unittests	92.79% <100.00%> (+0.01%)	⬆️

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[bblommers]

Thanks @viren-nadkarni!