return the cancellation cause for cancelled dials

[marten-seemann]

Fixes #3143.

[codecov]

Codecov Report

Merging #4078 (8646094) into master (abfe1ef) will increase coverage by 0.01%.
Report is 1 commits behind head on master.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #4078      +/-   ##
==========================================
+ Coverage   83.42%   83.42%   +0.01%     
==========================================
  Files         147      147              
  Lines       14888    14894       +6     
==========================================
+ Hits        12419    12425       +6     
  Misses       1972     1972              
  Partials      497      497              

Files Changed	Coverage Δ
client.go	58.12% <0.00%> (ø)
packet_packer.go	83.50% <100.00%> (+0.16%)	⬆️