Adapt use of ERROR_OUTPUT

[ssiccha]

ERROR_OUTPUT was made a read-only global variable. But we still want to change it.

[codecov[bot]]

Codecov Report

Merging #85 into master will decrease coverage by 0.16%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
- Coverage   72.13%   71.97%   -0.17%     
==========================================
  Files          14       14              
  Lines         865      860       -5     
==========================================
- Hits          624      619       -5     
  Misses        241      241

Impacted Files	Coverage Δ
gap/JupyterKernel.gi	80.53% <100%> (+0.14%)	:arrow_up:
gap/JupyterStream.gi	70% <0%> (-0.43%)	:arrow_down:
gap/JupyterHelp.gi	48.1% <0%> (-0.28%)	:arrow_down:
gap/JupyterInspection.gi	87.32% <0%> (-0.18%)	:arrow_down:
gap/JupyterStream.gd	100% <0%> (ø)	:arrow_up:
gap/JupyterUtil.gd	100% <0%> (ø)	:arrow_up:
gap/JupyterHelp.gd	100% <0%> (ø)	:arrow_up:
gap/JupyterRenderable.gd	100% <0%> (ø)	:arrow_up:

[embray]

Looks good, thanks.