feat: better error diagnostics

[gilescope]

A mechanism for being more explicit about what the causes of an error might be.

[gemini-code-assist]

Code Review

This pull request introduces an exitCodeDiagnostic helper function to provide detailed troubleshooting tips when a command fails with exit code 126, and includes unit tests to verify this functionality. The feedback suggests refactoring the conditional block in main.go to avoid using an if statement with an initializer, declaring the diagnostic variable on a separate line instead.

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[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	5328
This PR	5328
Difference	+0

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.