Resolve Miscellaneous Bugs in Beacon Node

[rauljordan]

This PR is a collection of fixes and small improvements to close out open bug reports.

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Description

Write a summary of the changes you are making

• Check for nil head block in rpc.GetChainHead(), resolves Error message at beacon chain start up when running eth2stats #4605
• Turn various scary ERROR level logs to WARN or DEBUG level, resolves ERROR sync: Failed to decode stream message error=i/o deadline reached peer=16Uiu2HAmDiKH4H8hpHDjhad69qVkTPzCVTvaz7VvBgzNTypz4uqk topic=/eth2 #4591 and resolves Change a couple "ERROR" messages to "WARN" instead #4652
• Turn powchain EOF error into WARN level, as the powchain service attempts to reconnect always - resolves ERROR powchain: Subscription to new head notifier failed error=websocket: close 1006 (abnormal closure): unexpected EOF #4517

[codecov]

Codecov Report

Merging #4743 into master will increase coverage by 33.85%.
The diff coverage is 25%.

@@             Coverage Diff             @@
##           master    #4743       +/-   ##
===========================================
+ Coverage   12.26%   46.12%   +33.85%     
===========================================
  Files          77      205      +128     
  Lines        6260    15437     +9177     
===========================================
+ Hits          768     7120     +6352     
- Misses       5360     7143     +1783     
- Partials      132     1174     +1042