[MSHARED-1420] Emit warn in case the embedded launcher cannot be used

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MSHARED-1420](https://issues.apache.org/jira/browse/MSHARED-1420?redirect=false)** and commented

Currently if some class loading exception happens while trying to initialize the embedded launcher it will transparent fall back to fork mode without emitting anything (https://github.com/apache/maven-verifier/blob/ef9d6fb01132b91998683268d51a75c9aca8e4b8/src/main/java/org/apache/maven/shared/verifier/Verifier.java#L1168). There should be at least a warning being emitted to clarify why the embedded launcher was not used.

This is particularly an issue from within an IDE where `mvn` cannot be found in the PATH.

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**Affects:** maven-verifier-1.8.0

Start at src/main/java/org/apache/maven/shared/verifier/Verifier.java around line 1168, where the embedded launcher fallback is handled. Trace the class-loading exception path and verify that a warning explains why fork mode was selected, while the existing fallback still works.

Written by the indexing model from the issue text.