Better error messages when koans are run outside of the "koans" directory #61
Description
This term, a student was rightfully surprised when they ran mvnw exec:java from the top-level directory of the GitHub checkout and encountered this error:
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:3.5.0:java (default-cli) on project koans: An exception occurred while executing the Java class. null: ExceptionInInitializerError: java.io.FileNotFoundException: /Users/david/PSU/git/DavidWhitlock/koans/app/config/i18n/messages_en.properties (No such file or directory) -> [Help 1]
It would be better to provide an error that indicates that the koans should be run from the "koans" directory.