[MNG-7492] Invalid POMs are blithefully ignored

**[Robert Krajewski](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=krajewski)** opened **[MNG-7492](https://issues.apache.org/jira/browse/MNG-7492?redirect=false)** and commented

Invalid poms prevent construction of the model's dependencies tree (and thus the model itself) and yet are only warnings:

`[WARNING] Invalid POM xxxx, transitive dependencies (if any) will not be available, enable debug logging for more details`

Once this happens, Java compiler errors often ensue and are completely mysterious. Debug logging will tell you why the POM is missing, but it doesn't stop a nonsensical model from being treated as valid. Which it's {**}not{**}.

See https://stackoverflow.com/questions/59944898/fail-on-invalid-or-missing-poms for a discussion. Workarounds don't fix the issue.

Please implement an option to make this situation fail the build in Maven 3. Even better, make it the default in a future version like Maven 4.

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**Affects:** 3.6.3

**Backported to:** [wontfix-candidate](https://github.com/apache/maven/milestone/119?closed=1)