refactor: throw informative exception when sniper printing cannot be done

[slarse]

Fix #3760

See #3760 for a description of the problem. I think it's more work than it's worth to try to allow for sniper printing of a subset of a compilation unit's types. There's an initial problem in that DJPP clones the CU if it discovers that one is trying to print only a subset of the types (this is where the current crash described in #3760 comes from). Even working around that, there's still a bit of trickery involved in printing just a subset of the types. A couple more hours on this and I could probably solve it, but I just don't think it's worth it.

This PR just throws a more informative exception that looks like this:

java.lang.IllegalArgumentException: Can only sniper print exactly all declared types of the compilation unit. Given types: [MultipleTopLevelTypes]. Declared types: [OtherTopLevelType, MultipleTopLevelTypes, TopLevelInterface]

I'm not quite sure if "fix" is the correct label for this PR, feel free to re-label.

[monperrus]

LGTM. Will merge.

[slarse]

@monperrus Ping for merge? :)

[monperrus]

Thanks Simon. Good informative exceptions are essential for a good user experience.