Use warning messages for connection issues

[t-bast]

lightning/bolts#834 recommends sending warning messages instead of connection-level errors in some cases, which avoids unnecessary channel closure.

[codecov-commenter]

Codecov Report

Merging #1863 (3c7c0ee) into master (4ca5c62) will decrease coverage by 0.03%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #1863      +/-   ##
==========================================
- Coverage   88.96%   88.92%   -0.04%     
==========================================
  Files         150      150              
  Lines       11250    11254       +4     
  Branches      457      467      +10     
==========================================
  Hits        10008    10008              
- Misses       1242     1246       +4     

Impacted Files	Coverage Δ
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.19% <0.00%> (-0.09%)	⬇️
...-core/src/main/scala/fr/acinq/eclair/io/Peer.scala	89.52% <0.00%> (-0.06%)	⬇️
...main/scala/fr/acinq/eclair/io/PeerConnection.scala	84.16% <57.14%> (-0.52%)	⬇️
...main/scala/fr/acinq/eclair/router/Validation.scala	90.76% <0.00%> (-1.54%)	⬇️
.../acinq/eclair/blockchain/bitcoind/ZmqWatcher.scala	96.87% <0.00%> (+0.78%)	⬆️
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	97.63% <0.00%> (+1.57%)	⬆️

[pm47]

I see what you did there! 😉