fix(injector: config): better error message when orchestrion.tool.go is missing

[eliottness]

Better error messages for cases like #535

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.80%. Comparing base (0ef21f0) to head (55d3d31).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/jobserver/buildid/version.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #536      +/-   ##
==========================================
- Coverage   67.87%   66.80%   -1.07%     
==========================================
  Files         104      104              
  Lines        6733     6736       +3     
==========================================
- Hits         4570     4500      -70     
- Misses       1697     1792      +95     
+ Partials      466      444      -22     

Components	Coverage Δ
Generators	46.17% <ø> (ø)
Instruments	∅ <ø> (∅)
Go Driver	75.89% <ø> (ø)
Toolexec Driver	58.49% <ø> (ø)
Aspects	75.61% <ø> (ø)
Injector	75.09% <100.00%> (+0.35%)	⬆️
Job Server	67.39% <0.00%> (-3.71%)	⬇️
Other	66.80% <83.33%> (-1.07%)	⬇️

Files with missing lines	Coverage Δ
internal/injector/config/config-go.go	75.49% <100.00%> (+10.84%)	⬆️
internal/jobserver/buildid/version.go	72.18% <0.00%> (ø)

... and 2 files with indirect coverage changes