[BUG][JAVA] Gradle plugin ignores "templateDir"

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

The Gradle plugin seems to ignore the templateDir argument set in build.gradle: the argument points to a non-existent directory but no exception is thrown when calling openApiGenerate (whereas a non-existent file as the inputSpec argument does throw).

openapi-generator version

Gradle plugin 4.2.3

OpenAPI declaration file content or url

openapi.yaml

Steps to reproduce

1. git clone git@github.com:rechvs/openapi-generator-gradle-plugin-debugging.git
2. ./gradlew openApiGenerate
3. No exception is thrown, despite templateDir pointing to a non-existent directory.

Start with the linked build.gradle and openapi.yaml, then run ./gradlew openApiGenerate from the reproduction project. Trace how the Gradle plugin handles the templateDir argument and compare it with inputSpec validation; done means a nonexistent templateDir is reported instead of being silently ignored.