Improve the error message when echo tests fail on missing target executable

[sbergen]

If you don't have an executable required by the echo tests installed, you'd get a somewhat cryptic error along the lines of:

thread 'tests::echo::echo_dict' panicked at test-output/src/tests/echo.rs:74:10:
spawn run process: Os { code: 2, kind: NotFound, message: "No such file or directory" }

It will now be this instead (if you don't have bun installed):

thread 'tests::echo::echo_dict' panicked at test-output/src/tests/echo.rs:74:29:
Failed to spawn process 'bun': No such file or directory (os error 2)

[lpil]

Thank you