Warning instead of error when failing to write tracking results

Summary

Error while writing tracking result into files does not end up as an error, but as a warning.

Steps to reproduce

Remove writing rights for an element file's directory.

chmod 0555 elements

Then run track.

What is the current bug behavior?

Prints:

[00:00:00][][] SUCCESS Track

Note: The file has not been modified, which is fine.

What is the expected correct behavior?

Should print:

[00:00:00][][] FAILURE Track

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.1

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