Remove EnsembleMissingPipelinesError exception

[angela97lin]

Closes #2954

[codecov]

Codecov Report

Merging #3131 (45ec93a) into main (eb1e3e0) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3131     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        317     317             
  Lines      30718   30711      -7     
=======================================
- Hits       30610   30607      -3     
+ Misses       108     104      -4     

Impacted Files	Coverage Δ
evalml/exceptions/__init__.py	100.0% <ø> (ø)
evalml/exceptions/exceptions.py	100.0% <ø> (ø)
evalml/pipelines/utils.py	99.7% <ø> (ø)
evalml/tests/component_tests/test_components.py	99.2% <100.0%> (+0.3%)	⬆️
evalml/utils/gen_utils.py	98.7% <100.0%> (+1.0%)	⬆️

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[freddyaboulton]

Looks good to me @angela97lin !!