fix electrum resolve_height for offchain TXs

[zoedberg]

In the PR #198, specifically in the commit eeb9d55, the refactor removed an important part in the resolve_height method of the electrum resolver. The check of No such mempool or blockchain transaction error was there to detect offchain transactions. I've re-added the check so that know the behavior of the electrum resolver is the same of the esplora one.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Project coverage is 0.0%. Comparing base (062964a) to head (2cd242a).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/resolvers/electrum_blocking.rs	0.0%	10 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##           master   #214   +/-   ##
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  Coverage     0.0%   0.0%           
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  Files          10     10           
  Lines         913    924   +11     
=====================================
- Misses        913    924   +11     

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[dr-orlovsky]

One small stylistic improvement, otherwise looks good

[dr-orlovsky]

ACK 2cd242a