handle every error

[Guiorgy]

Currently, handle_error, and thus the VACKUP_FAILURE_SCRIPT script as well, is only called if an executed command exits with an error. They are not called when the script exits with its own error, for example: echo "Error: Volume $VOLUME_NAME does not exist"; exit 1. This PR adds a new error function that ensures the handle_error function is called at the end with the correct caller line number, and changes every exit 1 call in the script to error instead.

Also fixed the inconsistency in the "Volume $VOLUME_NAME does not exist, creating..." message and changed it into a wanring instead of an error:

echo "Error: Volume $VOLUME_NAME does not exist" # old
echo "Warning: Volume $VOLUME_NAME does not exist, creating..." # new

and

echo "Volume $VOLUME_NAME does not exist, creating..." # old
echo "Warning: Volume $VOLUME_NAME does not exist, creating..." # new

[Guiorgy]

Also, FYI the workflow is failing with the following message:

[Invalid workflow file: .github/workflows/linter.yml#L14](https://github.com/BretFisher/docker-vackup/actions/runs/7829440253/workflow)
error parsing called workflow ".github/workflows/linter.yml" -> "bretfisher/super-linter-workflow/.github/workflows/super-linter.yaml@main" : failed to fetch workflow: workflow was not found.