🐛 FIX: Inline exec variables with multiple outputs

[chrisjsewell]

Previously, it was assumed that a variable evaluation would only ever create 0 or 1 outputs.
Multiple are now allowed.
Also, error messages are appended with the output index.

closes #436

[codecov]

Codecov Report

Base: 81.40% // Head: 81.37% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (9cd6070) compared to base (392221e).
Patch coverage: 79.41% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #440      +/-   ##
==========================================
- Coverage   81.40%   81.37%   -0.03%     
==========================================
  Files          29       29              
  Lines        2560     2572      +12     
==========================================
+ Hits         2084     2093       +9     
- Misses        476      479       +3     

Flag	Coverage Δ
pytests	81.37% <79.41%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
myst_nb/docutils_.py	74.26% <0.00%> (-0.44%)	⬇️
myst_nb/core/variables.py	74.11% <72.72%> (+0.43%)	⬆️
myst_nb/ext/eval/__init__.py	74.16% <84.61%> (+0.43%)	⬆️
myst_nb/core/execute/base.py	97.70% <100.00%> (ø)
myst_nb/core/execute/inline.py	79.62% <100.00%> (ø)
myst_nb/ext/glue/directives.py	68.46% <100.00%> (ø)
myst_nb/ext/glue/roles.py	47.82% <100.00%> (ø)

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