SUP-1372 fix compressed

[HugeIRL]

This PR adds a check to the compressed: setting to make sure the file or directory exists before attempting to compress. It will also not attempt to compress if the file/folder is either missing or empty, but display an error message about being unable to compress, and then continue on running instead of outright failing.

A new test has been added to check for path existence on compressed: for the expected graceful error and continue message.

A new test has been added to check for path existence on compressed when ignore-missing is set for the expected graceful error and continue message.

The rest of the affected failing tests have also been updated to create each tests mock file and delete it after the test is done so the tests no longer fail due to the new existence check.

This will fix #89

[pzeballos]

In the tests, you don't need to create a file *.log (and remove it later). That is actually an issue with the plugin that glob paths are not supported with compressed. So in this case because you are changing the logic, these tests started to fail. Instead of creating that file, you can simply update the variable of the test to use a particular file (like we have in the docs), for example:
export BUILDKITE_PLUGIN_ARTIFACTS_UPLOAD="file.log"

We also need to add a test when the ignore-missing is set and the file doesn't exist

[pzeballos]

Looking good! I added those two suggestions and did a small change in the docs 🥳