when(project.getArtifactId()).thenReturn("maven-embedder");
        when(project.getPackaging()).thenReturn("jar");
        when(project.getName()).thenReturn("Apache Maven Embedder");
        when(project.getVersion()).thenReturn("3.5.4-SNAPSHOT");
        when(project.getFile()).thenReturn(new File(basedir, "maven-embedder/pom.xml"));
        when(event.getProject()).thenReturn(project);

package org.apache.maven.embedder;

/**
 * Hello world!
 *
 */
public class App
{
    public static void main( String[] args )
    {
        System.out.println( "Hello World!" );
    }

package org.apache.maven.embedder;

import junit.framework.Test;
import junit.framework.TestCase;
import junit.framework.TestSuite;

/**
 * Unit test for simple App.
 */
public class AppTest
    extends TestCase
{
    /**
     * Create the test case
     *
     * @param testName name of the test case
     */
    public AppTest( String testName )
    {
        super( testName );
    }

    /**

    <module>maven-settings</module>
    <module>maven-settings-builder</module>
    <module>maven-artifact</module>
    <module>maven-resolver-provider</module>
    <module>maven-repository-metadata</module>
    <module>maven-embedder</module>
    <module>maven-compat</module>
    <module>maven-toolchain-model</module>
    <module>maven-toolchain-builder</module>
  </modules>

  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.apache.maven.embedder</groupId>
  <artifactId>simple-project</artifactId>
  <version>1.0-SNAPSHOT</version>
  <name>simple-project</name>
  <url>http://maven.apache.org</url>
  <dependencies>
    <dependency>

    <groupId>org.apache.maven</groupId>
    <artifactId>maven</artifactId>
    <version>4.0.0-beta-6-SNAPSHOT</version>

    <relativePath>../../</relativePath>
  </parent>

  <artifactId>maven-embedder</artifactId>

  <name>Maven Embedder</name>
  <description>Maven embeddable component, with CLI and logging support.</description>

  <dependencies>
    <dependency>
      <groupId>org.apache.maven</groupId>

    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-api-cli</artifactId>
    </dependency>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-embedder</artifactId>
    </dependency>
    <dependency>
      <groupId>org.codehaus.plexus</groupId>
      <artifactId>plexus-classworlds</artifactId>
    </dependency>
    <dependency>
      <groupId>javax.inject</groupId>

   * The HTTP <a href="https://wicg.github.io/cross-origin-embedder-policy/#COEP">{@code
   * Cross-Origin-Embedder-Policy}</a> header field name.
   *
   * @since 30.0
   */
  public static final String CROSS_ORIGIN_EMBEDDER_POLICY = "Cross-Origin-Embedder-Policy";
  /**
   * The HTTP <a href="https://wicg.github.io/cross-origin-embedder-policy/#COEP-RO">{@code
   * Cross-Origin-Embedder-Policy-Report-Only}</a> header field name.
   *

  <dependencies>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-cli</artifactId>
    </dependency>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-embedder</artifactId>
    </dependency>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-core</artifactId>
    </dependency>
    <dependency>
      <groupId>org.apache.maven</groupId>

  project
- nice little sample projects that could be run in the core as well as integration tests

All Possible Errors
- invalid lifecycle phase (maybe same as bad CLI param, though you were talking about embedder too)
- <module> specified is not found
- malformed settings
- malformed POM
- local repository not writable
- remote repositories not available
- artifact metadata missing
- extension metadata missing