remote/client: cleanup InteractiveCommandError

[sgross-emlix]

When handling InteractiveCommandError there is an implicit assumption that it has a field exitcode.
This assumption holds since in all places it is raised there is the same repeated code block.

Make this assumption more explicit by providing a proper constructor to InteractiveCommandError class.

[sgross-emlix]

Moved out of #1769 as suggested by @jluebbe

[codecov]

Codecov Report

❌ Patch coverage is 10.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.1%. Comparing base (a175bbc) to head (a21de65).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/remote/client.py	10.0%	9 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1788   +/-   ##
======================================
  Coverage    45.0%   45.1%           
======================================
  Files         172     172           
  Lines       13630   13618   -12     
======================================
  Hits         6144    6144           
+ Misses       7486    7474   -12     

Flag	Coverage Δ
3.10	45.1% <10.0%> (+<0.1%)	⬆️
3.11	45.1% <10.0%> (+<0.1%)	⬆️
3.12	45.1% <10.0%> (+<0.1%)	⬆️
3.13	45.0% <10.0%> (+<0.1%)	⬆️
3.14	45.0% <10.0%> (+<0.1%)	⬆️
3.9	45.1% <10.0%> (+<0.1%)	⬆️

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