ElectrumWatcher: fix for unconfirmed txs

[t-bast]

Electrum can return unconfirmed txs in an address' history. When that happens, we should not try to fetch its confirmed position, it will return an error.

We simply need to ignore these events and wait for the tx to confirm.

I'll cherry-pick this commit to the android and android-phoenix branches once it's on master.

[codecov-io]

Codecov Report

Merging #1647 (d57545c) into master (9425fd4) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1647      +/-   ##
==========================================
+ Coverage   85.92%   85.99%   +0.06%     
==========================================
  Files         151      151              
  Lines       11452    11465      +13     
  Branches      512      490      -22     
==========================================
+ Hits         9840     9859      +19     
+ Misses       1612     1606       -6     

Impacted Files	Coverage Δ
...nq/eclair/blockchain/electrum/ElectrumClient.scala	74.26% <ø> (+0.73%)	⬆️
...q/eclair/blockchain/electrum/ElectrumWatcher.scala	75.39% <100.00%> (+0.79%)	⬆️
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	93.70% <0.00%> (-1.58%)	⬇️
...in/scala/fr/acinq/eclair/channel/Commitments.scala	93.28% <0.00%> (+0.05%)	⬆️
...main/scala/fr/acinq/eclair/io/PeerConnection.scala	82.35% <0.00%> (+0.42%)	⬆️
...ain/scala/fr/acinq/eclair/payment/Monitoring.scala	98.11% <0.00%> (+0.49%)	⬆️
...clair/blockchain/electrum/ElectrumClientPool.scala	78.49% <0.00%> (+4.30%)	⬆️