Allow returning error codes when running scripts headlessly

When executing ImageJ scripts, plugins or macros headlessly, it would be nice if the process could give a non-zero return code in case of error. Currently, ImageJ1 does not do this.

Added by @luxigo:

> It would be helpful if Fiji exit code was not zero when an unhandled exception (eg: java.awt.HeadlessException) is raised in headless mode... at least optionally.

Start by tracing how headless ImageJ scripts, plugins, and macros are executed and how the process currently handles unhandled exceptions. Define and verify an optional non-zero exit code for failures, including a headless exception, while preserving the existing behavior when the option is not enabled.

Written by the indexing model from the issue text.