Fix panic when error is not *exec.ExitError (Fixes #239)

[jwilder]

This pull request improves error handling in command execution by making the extraction of exit codes from errors more robust and by adding targeted unit tests for this logic. The changes ensure that exit codes are handled correctly for different error types and that the behavior is well-tested.

Error handling improvements:

• Refactored the exit code extraction logic in runCmd (in exec.go) to safely handle errors that may not be of type *exec.ExitError, using errors.As for type assertion and providing a fallback exit code of 1 for non-standard errors.
• Added the errors package import to support the improved error handling.

Testing enhancements:

• Added a new file exec_error_test.go with unit tests for exit code extraction, including cases for both standard *exec.ExitError and generic errors, and introduced a helper function extractExitCode mirroring the runtime logic.

Fixes #239