Parse RPC errors as JSON content

[jkczyz]

Bitcoin Core's JSON RPC server returns errors as HTTP error responses with JSON content in the body. Parse this content as JSON to give a more meaningful error. Otherwise, "binary" is given because the content contains ASCII control characters.

Most of the parsing was already in place, but it was only triggered for OK responses.

[codecov]

Codecov Report

Merging #929 (b2f16ad) into main (f8450a7) will increase coverage by 1.70%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##             main     #929      +/-   ##
==========================================
+ Coverage   90.42%   92.13%   +1.70%     
==========================================
  Files          59       61       +2     
  Lines       30173    37142    +6969     
==========================================
+ Hits        27285    34222    +6937     
- Misses       2888     2920      +32     

Impacted Files	Coverage Δ
lightning-block-sync/src/http.rs	93.72% <75.86%> (-1.28%)	⬇️
lightning-block-sync/src/rpc.rs	78.51% <84.61%> (+0.13%)	⬆️
lightning/src/util/ser_macros.rs	93.61% <0.00%> (-3.12%)	⬇️
lightning/src/ln/mod.rs	89.28% <0.00%> (-0.72%)	⬇️
lightning/src/chain/mod.rs	100.00% <0.00%> (ø)
lightning/src/chain/onchaintx.rs	93.68% <0.00%> (ø)
lightning/src/chain/package.rs	92.32% <0.00%> (ø)
lightning/src/util/ser.rs	93.18% <0.00%> (+0.20%)	⬆️
lightning/src/ln/functional_tests.rs	97.75% <0.00%> (+0.68%)	⬆️
lightning/src/util/enforcing_trait_impls.rs	91.27% <0.00%> (+0.89%)	⬆️
... and 8 more

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[ariard]

Good to me b2f16ad