Add traceback to error 500 page

[benjaoming]

Summary

Due to various issues reported, I'm trying to affirm that device registration works through a bit of fumbling.

• Add an additional test case to clear that online presence is correctly detected.
• Add a proper error traceback so we can find out what the initial reports were about (since logs had no info)

Issues addressed

5401

5123

[codecov-io]

Codecov Report

Merging #5405 into 0.17.x will increase coverage by 0.04%.

@@            Coverage Diff            @@
##           0.17.x   #5405      +/-   ##
=========================================
+ Coverage   51.76%   51.8%   +0.04%     
=========================================
  Files         143     143              
  Lines        7499    7499              
=========================================
+ Hits         3882    3885       +3     
+ Misses       3617    3614       -3

Impacted Files	Coverage Δ
kalite/distributed/views.py	61.14% <ø> (+1.91%)	:white_check_mark:

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[benjaoming]

New error page (before clicking textarea)

After clicking text area:

[radinamatic]

:heart_eyes:

[benjaoming]

Merging this, because then at least we can know about the other issue on Mac/Windows that we haven't been able to troubleshoot through logs etc.