polus plugins : Improve error messages for end users

[agerardin]

Issue for polus plugins

Summary

Error messages are not always helpful for the end user.

What is the expected correct behavior?

Error messages during validation should be a easy as possible to understand for the end user to guide him towards fixing its submission.

Steps to reproduce

Ex: submit a bad manifest (polus.plugins.submit_plugin(bad_manifest) )

Relevant screenshots and/or logs

Possible fixes

We can add some acceptance tests for most common mistakes user make so that we can check we provide valid message. Ex : check we have well-formed json first so if the user messed up his submission and submit a non-json file, he knows it right away.