Web app authors can be very creative, but one thing I don't like about web apps is that often their error handling is nonexistent. It's not about individual authors; behemots like Microsoft also do this in an irritating way: "Something did not work. Reload the page please" -- Is this really the best a billion dollar company can do? Well, I suppose this is still better than no error at all, which is a default error handling method for big chunk of web apps.
I think (but can be wrong) that by using such a dynamic environment like JS is the culprit for lack of good error checking mechanism.
Also one thought of mine for the reason why even big companies are unwilling to provide good error messages is that it's impossible to get the real reason. There may be so many moving parts that even the developer isn't able to handle it all. Which sparks a doubt in my mind if this environment is really fit enough to provide high quality software. Since it's so popular it's obvious that it has some aspects that it does right, but it seems that the web app environment could benefit from some fundamental changes.