Okay, in ce7fb7d7
I've changed the error message to be only displayed when the plugin is actually loaded. This means that it will be displayed less often (e.g. only when a page is rendered for syntax plugins) but now it is really only displayed when loading a plugin class failed because the plugin name is not valid (which means now that it actually can't be part of a valid PHP class name, we could change this later to be actually only [a-z0-9]+ as explained in the plugin documentation but this breaks plugins like iCalendar).
Concerning the plugin list: I think the problem is that it is difficult to check if a certain directory is a plugin as plugin.info.txt files aren't in every plugin and apart from that we would need to check for all different plugin types which might slow down page loading as for example it makes no sense to check for admin or syntax plugins when a cached page is displayed. However this could be done in the plugin manager but I don't think we should implement that in the current plugin manager but this is rather a feature for the new extension manager (if it hasn't been implemented already). I think it makes sense to show the directory entry but to show that it doesn't contain any DokuWiki plugin data.