core: fix contract's state migration and don't swallow errors

[roman-khimov]

Fixes difference in state changes at mainnet's block 2442790 because contract
migration in b4eb2dc35226e6520ee4e09a56197dff91547b50a7f57edc82930fc18c75dffc
doesn't actually transfer the storage state, it only deletes the old one.

And add an error check just in case.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ba1c3bf). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master     #760   +/-   ##
=========================================
  Coverage          ?   64.77%           
=========================================
  Files             ?      139           
  Lines             ?    12736           
  Branches          ?        0           
=========================================
  Hits              ?     8250           
  Misses            ?     4085           
  Partials          ?      401

Impacted Files	Coverage Δ
pkg/core/interop_neo.go	59.03% <0%> (ø)

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