Change void_result to error_result

[jenow]

What does this PR do ?

fix #148

Rename void_result to error_result as it is weird to have a struct named void_result which contains something (errors).
The error_result will be null if no errors are received otherwise it will be allocated and contain the status, error and stack.

How should this be manually tested?

See travis report.

[codecov-io]

Codecov Report

Merging #150 into 1.x will increase coverage by 0.09%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             1.x   #150      +/-   ##
=======================================
+ Coverage   88.9%    89%   +0.09%     
=======================================
  Files        244    244              
  Lines       4382   4382              
=======================================
+ Hits        3896   3900       +4     
+ Misses       455    451       -4     
  Partials      31     31

Impacted Files	Coverage Δ
realtime/join.go	78.37% <0%> (+5.4%)	⬆️
realtime/subscribe.go	77.14% <0%> (+5.71%)	⬆️

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