Cleaned up SandboxedError exception message and tried to make it clearer

I'm not sure if this is an issue of people not reading past the last line, or that the error message may have seemed to imply that the second half of the message was the cause of the problem, or more likely a combination of the two, but there have been a bunch of people reporting this error message as if it was the root problem, sometimes without any other context.

Unfortunately this may not be entirely unambiguous, since the message is now long enough that it can't really fit on one line, so saying that the root cause is probably directly above this could be misunderstood, but I'm not sure how to improve on it and this should be better than before.