Check valid and existing versions in webservices

[jpmckinney]

From #421

version is already handled on eggstorage side, but we could also raise invalid request 400 HTTP code on invalid versions

We can do this using IEggStorage.get(). If (None, None) is returned, then no version matches. (That branch only runs if no version is provided.)

[jpmckinney]

#500 has some relevant logic (project_version_exists) for checking whether the version is found, which is maybe more useful than just the validation.

[jpmckinney]

Like with #117, I don't think we need to validate the version in the API.

I think it has never happened that a user configures an invalid version (e.g. containing "/", causing directory descent) or a colliding version (e.g. using the characters that eggstorage replaces with underscores).

As mentioned in #421, we can't change status codes without breaking backwards compatibility.

[jpmckinney]

In terms of checking that the project and/or version exists:

• schedule.json
• listspiders.json
• delversion.json
• delproject.json