Better error messages when koans are run outside of the "koans" directory

[DavidWhitlock]

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.

[DavidWhitlock]

Oh, interesting. I'm not sure if that error actually occurred when ./mvnw exec:java was run in the top-level directory.

When I do that, I see:

[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:3.6.3:java (default-cli) on project JoyOfCodingWinter2026: The parameters 'mainClass' for goal org.codehaus.mojo:exec-maven-plugin:3.6.3:java are missing or invalid -> [Help 1]

It still might be confusing to the students, though.