Some others might have better responses on this, but my first thought is that some point you are going to run into your question being a matter of biology rather than philosophy. Philosophically, the issue divides between the mind (whatever that is) and the eyes (for example, whatever that is) and the formation of opinions. Philosophy isn't going to be able to tell you the exact mechanism of these things, but when you are asking mental questions about whether to trust "the senses" or "something else" or "nothing at all" then you into philosophical choices. Reductionism to worrying about what particular atoms are doing at a particular moment is impossible and self-defeating and not required in order to come up with a logical framework of analysis. I think Epicurus was mostly concerned with answering those from a philosophical perspective who asserted that "the senses can't be trusted" but on the other hand "reason" or "logic" or "the gods" can be trusted. Epicurus is pointing out that those alternatives do not really exist, in that the gods don't answer these questions for you, and "logic" and "reason" do not have direct connections with reality to answer them either. Only the senses (and anticipations and feelings) are considered to be direct connections with reality whose input must be dealt with as "real."
Also as to this
I hadn't considered the mind and the senses as being separate, and error in the former being caused by lack of information from the latter
It would probably be better to say that the mind and the senses are separate, and that errors in the mind are caused by misinterpretation by the mind of the information provided by the senses. The senses report "truly" exactly what they perceive, without any added opinion. Exactly what to make of what they receive, however, is the job of the mind, and the mind very easily jumps to conclusions that are not warranted by specific perceptions of the senses. The only way to judge the 'truth" of an opinion generated from one set of observations is to test it by the opinions generated by many other observations over time and from various perspectives. Only when the assembled opinions start to resolve into a consistent conclusion are we confident that the opinion is correct. So sometimes "lack of information" would be the right term, but probably a better way of looking at it is that the observations over time are not consistent, so we need to suspect that something is going on to cause us to want to "wait" to form a strong opinion until the observations begin to resolve consistently with observations about which we are confident.