If the source directory is empty, the following error is thrown ``` Caused by: java.lang.UnsupportedOperationException: Empty collection can't be reduced. at lang.taxi.Compiler.compile(Compiler.kt:390) at lang.taxi.cli.commands.BuildCommand.loadSources(BuildCommand.kt:53) at lang.taxi.cli.commands.BuildCommand.execute(BuildCommand.kt:23) at lang.taxi.cli.ShellRunner.run(ShellRunner.kt:41) at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:818) ... 4 common frames omitted ``` Should be a warning, and exit gracefully