fix: return error on invalid ref config

[jahvon]

This "fixes" a panic that happens if the cmd or ref isn't defined for a serial or parallel exec

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 34.62%. Comparing base (fd5bbc7) to head (61dfbb9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/runner/parallel/parallel.go	0.00%	2 Missing ⚠️
internal/runner/serial/serial.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #186      +/-   ##
==========================================
- Coverage   34.64%   34.62%   -0.03%     
==========================================
  Files          81       81              
  Lines        6742     6746       +4     
==========================================
  Hits         2336     2336              
- Misses       3968     3974       +6     
+ Partials      438      436       -2     

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