Improve the error message when the build data file has been deleted

[edmorley]

On Honeycomb I've seen cases of builds failing due to a pre/post-compile hook having deleted the buildpack's internal build data file - typically as part of the user trying to clean up the build cache.

This file is required for the buildpack to function correctly, so deleting it isn't supported. However, we can show a clearer error message.

Before:

-----> Running bin/post_compile hook
       + rm -rf /tmp/cache
jq: error: Could not open file /tmp/cache/build-data/python.json: No such file or directory
jq: error: Could not open file /tmp/cache/build-data/python.json: No such file or directory

 !     Internal Error: An unhandled buildpack error occurred.
 !
 !     An unhandled error occurred while executing the command:
 !     jq --exit-status --arg key "${key}" "${jq_args[@]}" '. + { ($key): ($value) }' "${BUILD_DATA_FILE}"
 !
 !     If this issue persists, please open a support ticket or file
 !     an issue on the buildpack's GitHub repository:
 !     https://help.heroku.com/
 !     https://github.com/heroku/heroku-buildpack-python/issues
 !
 !     Stack trace:
 !     build_data::_set @ /tmp/buildpack/lib/build_data.sh:109
 !     build_data::set_raw @ /tmp/buildpack/lib/build_data.sh:76
 !     build_data::set_duration @ /tmp/buildpack/lib/build_data.sh:60
 !     hooks::run_hook @ /tmp/buildpack/lib/hooks.sh:37
 !     main @ ./bin/compile:289

jq: error: Could not open file /tmp/cache/build-data/python.json: No such file or directory
jq: error: Could not open file /tmp/cache/build-data/python.json: No such file or directory

 !     Internal Error: An unhandled buildpack error occurred.
 !
 !     An unhandled error occurred while executing the command:
 !     new_data_file_contents="$(jq --exit-status --arg key "${key}" "${jq_args[@]}" '. + { ($key): ($value) }' "${BUILD_DATA_FILE}")"
 !
 !     If this issue persists, please open a support ticket or file
 !     an issue on the buildpack's GitHub repository:
 !     https://help.heroku.com/
 !     https://github.com/heroku/heroku-buildpack-python/issues
 !
 !     Stack trace:
 !     build_data::_set @ /tmp/buildpack/lib/build_data.sh:109
 !     build_data::set_raw @ /tmp/buildpack/lib/build_data.sh:76
 !     build_data::set_duration @ /tmp/buildpack/lib/build_data.sh:60
 !     hooks::run_hook @ /tmp/buildpack/lib/hooks.sh:37
 !     main @ ./bin/compile:289

jq: error: Could not open file /tmp/cache/build-data/python.json: No such file or directory

After:

-----> Running bin/post_compile hook
       + rm -rf /tmp/cache

 !     Error: The buildpack's build data file has been deleted.
 !
 !     The Python buildpack's build data file can't be found:
 !     /tmp/cache/build-data/python.json
 !
 !     This file is required for the buildpack to work correctly,
 !     and so you must not delete it when removing files from the
 !     build cache or /tmp directories.

Lastly, we also now use --exit-status in more places to prevent invalid empty build data files from being silently ignored.

GUS-W-20220731.