return error on SendMessage when session is closed

[mengelbart]

Returns the error from AddAndWait to the user of SendMessage e.g. when the session is closed.

[codecov]

Codecov Report

Merging #3218 (75d2276) into master (e9d12b7) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3218      +/-   ##
==========================================
+ Coverage   85.40%   85.41%   +0.01%     
==========================================
  Files         134      134              
  Lines        9825     9824       -1     
==========================================
  Hits         8391     8391              
+ Misses       1060     1059       -1     
  Partials      374      374              

Impacted Files	Coverage Δ
session.go	77.53% <0.00%> (+0.07%)	⬆️

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[marten-seemann]

Thank you!