Misleading error when no source

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