Fail with MojoExecutionException on malformed supplemental model (#266)

[elharo]

Closes #266

Test-first

Added RemoteResourcesMojoTest.testMalformedSupplementalModelFailsWithMojoExecutionException(), which configures a supplemental-models.xml containing a well-formed <project> element that the strict MavenXpp3Reader rejects (an unrecognized element).

The test asserts that executing the mojo fails with a MojoExecutionException whose message contains "Unable to parse supplemental XML".

Before the fix this test fails with:

java.lang.NullPointerException: Cannot invoke "org.apache.maven.model.Model.getGroupId()" because "m" is null

Fix

getSupplement() now throws a MojoExecutionException (with the parse cause) instead of logging a warning and returning null, so a malformed supplemental model entry fails the build cleanly rather than crashing with an NPE in loadSupplements().

Verification

• New test fails before the fix (NPE), passes after.
• Full suite: mvn verify passes (11 unit tests, RAT).