[MNG-8141] Do not loose model builder warnings

[cstamas]

Original change was merged but then reverted.

Changes in two areas:

• if model builder detects any "issue" with model being built (again, there may be NO validation at all, as for dependencies models are being built with MIN validation, basically NOTHING is validated). The change is essentially limited to Profiles, as "problems" were already collected by ModelBuilder
• artifact descriptor reader should not ignore WARNs as now, but report them. Before, ArtifactDescriptorReader was really tossing away WARNings from ModelBuilder, it reported them ONLY if Severity was FATAL or ERROR (as then ModelBuilder throws).

The goal is solely to make users aware, that their build pulls in dependencies (and dependencies of dependencies...) that have invalid POMs. This may affect build outcome, as for example inability to activate proper profile (ie. due duplicate profile IDs that IS caught by validation, but again, in this case deps-of-deps... we use MIN validation) may result in wrong dependendency set, missing properties and so on, as profile that was about to add these could not be activated.

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https://issues.apache.org/jira/browse/MNG-8141

[cstamas]

The reproducer now looks like this https://gist.github.com/cstamas/36903dd7701cbae94d5971211e835b05
In DEBUG is like this https://gist.github.com/cstamas/4ad6bbb6b7f3350dbf5439e397131803

And is good, goal is to raise awareness to a presence of invalid model in the build. Build still fails due MNG-8147

[cstamas]

Superseded by #1568