pulp ostree: composer should check pulp config before start to build image

[yih-redhat]

Describe the bug
When pulp enabled, composer will build image first and then upload image to pulp, this could cause a potential issue, that if customer provided wrong pulp information in pulp.toml, composer will still build the image first but will fail at upload step.
Composer should deny the request if pulp configuration is wrong, just like what it did for --url parameter.

Environment

• OS version (/etc/os-release and /etc/redhat-release):
• osbuild-composer version (rpm -qi osbuild-composer)

To Reproduce
Steps to reproduce the behavior:

• for example starting osbuild-composer.service or running composer-cli command, don't forget to include all configuration files you created

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
Add any other context about the problem here.

[achilleas-k]

The behaviour with the pulp uploader is the same as it is for all other uploaders/targets. Maybe this can be a more general change for verifying upload configurations before a build, if we want to go this way.