Improve diagnostics when forked JVM exits with non-zero status

Closes gh-16869

Author: wilkinsona

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RunMojo.java

@@ -69,17 +69,22 @@ protected void logDisabledFork() {
 	@Override
 	protected void runWithForkedJvm(File workingDirectory, List<String> args,
 			Map<String, String> environmentVariables) throws MojoExecutionException {
+		int exitCode = forkJvm(workingDirectory, args, environmentVariables);
+		if (exitCode == 0 || exitCode == EXIT_CODE_SIGINT) {
+			return;
+		}
+		throw new MojoExecutionException(
+				"Application finished with exit code: " + exitCode);
+	}
+
+	private int forkJvm(File workingDirectory, List<String> args,
+			Map<String, String> environmentVariables) throws MojoExecutionException {
 		try {
 			RunProcess runProcess = new RunProcess(workingDirectory,
 					new JavaExecutable().toString());
 			Runtime.getRuntime()
 					.addShutdownHook(new Thread(new RunProcessKiller(runProcess)));
-			int exitCode = runProcess.run(true, args, environmentVariables);
-			if (exitCode == 0 || exitCode == EXIT_CODE_SIGINT) {
-				return;
-			}
-			throw new MojoExecutionException(
-					"Application finished with exit code: " + exitCode);
+			return runProcess.run(true, args, environmentVariables);
 		}
 		catch (Exception ex) {
 			throw new MojoExecutionException("Could not exec java", ex);