[Bindgen Tool] Fix return statement generation for binding functions with error returns

[NipunaRanasinghe]

Purpose

This PR

• fixes Ballerina binding function generation logic for non-void java object methods which throws exceptions.
• updates the bindgen unit test suite to cover the aforementioned scenario.

Fixes https://github.com/ballerina-platform/ballerina-lang/issues/42624

Check List

• [x] Read the Contributing Guide
• [ ] Updated Change Log
• [x] Checked Tooling Support (#)
• [x] Added necessary tests
  • [x] Unit Tests
  • [ ] Spec Conformance Tests
  • [ ] Integration Tests
  • [ ] Ballerina By Example Tests
• [ ] Increased Test Coverage
• [ ] Added necessary documentation
  • [ ] API documentation
  • [ ] Module documentation in Module.md files
  • [ ] Ballerina By Examples