Improve handling when no MPI found.

[shuds13]

The MPI runner can be specified on the submit call or in an environment script, so should not error if not found before then.

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.37%. Comparing base (5bd1de2) to head (69f40cc).
Report is 13 commits behind head on develop.

Files with missing lines	Patch %	Lines
libensemble/executors/mpi_executor.py	0.00%	1 Missing and 1 partial ⚠️
libensemble/executors/mpi_runner.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1514      +/-   ##
===========================================
- Coverage    78.42%   78.37%   -0.06%     
===========================================
  Files           76       76              
  Lines         7690     7694       +4     
  Branches      1142     1144       +2     
===========================================
- Hits          6031     6030       -1     
- Misses        1465     1467       +2     
- Partials       194      197       +3     

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