Fix error msg to match jsonrpc method

[langep]

I think the error message should complain indicate failure to parse the "answer" instead of "offer" because this case handles an incoming answer.

[codecov]

Codecov Report

Merging #596 (576e334) into master (8d95f9f) will increase coverage by 0.68%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #596      +/-   ##
==========================================
+ Coverage   35.20%   35.89%   +0.68%     
==========================================
  Files          24       24              
  Lines        2619     2619              
==========================================
+ Hits          922      940      +18     
+ Misses       1558     1540      -18     
  Partials      139      139              

Impacted Files	Coverage Δ
pkg/sfu/router.go	46.19% <0.00%> (+3.50%)	⬆️
pkg/sfu/subscriber.go	56.41% <0.00%> (+7.69%)	⬆️

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[adwpc]

LGTM