Composer: Properly handle cases where `composer.json` is missing or malformed

Summary

When publishing a composer package, the composer.json file can:

1. be missing
2. the backend may be unable to parse it.

In both cases, the request will end up in a RuntimeError.

Steps to reproduce

• Publish a composer package where the composer.json file is missing or malformed.

What is the current bug behavior?

• The API request will end up with a RuntimeError. Our guess is that the backend will reply a 500 response.

What is the expected correct behavior?

• It would better if the API answers 400 Bad Request, meaning that the backend understood the request but given the parameters, it can't fulfill it.

Possible fixes

• Catch the possible errors here and answer a bad_request!