Better error message on extension import error

[mike0sv]

closes #200

[codecov]

Codecov Report

Merging #300 (76ad6a0) into main (b70975d) will decrease coverage by 2.14%.
The diff coverage is 32.00%.

@@            Coverage Diff             @@
##             main     #300      +/-   ##
==========================================
- Coverage   90.32%   88.18%   -2.15%     
==========================================
  Files          77       77              
  Lines        5698     5720      +22     
==========================================
- Hits         5147     5044     -103     
- Misses        551      676     +125     

Impacted Files	Coverage Δ
mlem/core/base.py	79.69% <8.33%> (-6.48%)	⬇️
mlem/core/errors.py	81.94% <33.33%> (-6.95%)	⬇️
mlem/ext.py	88.65% <100.00%> (+0.36%)	⬆️
mlem/contrib/docker/helpers.py	50.00% <0.00%> (-50.00%)	⬇️
mlem/contrib/heroku/utils.py	39.34% <0.00%> (-40.99%)	⬇️
mlem/contrib/heroku/meta.py	52.00% <0.00%> (-32.00%)	⬇️
mlem/contrib/heroku/build.py	56.75% <0.00%> (-27.03%)	⬇️
mlem/ui.py	85.50% <0.00%> (-8.70%)	⬇️
mlem/contrib/docker/base.py	81.72% <0.00%> (-5.38%)	⬇️
mlem/core/objects.py	86.46% <0.00%> (-4.99%)	⬇️
... and 5 more

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[aguschin]

LGTM!