I have come to the same problem and fixed the same way as Alex K.
So if "Send Errors To Browser" is not working set also this:
Error Pages -> 500 -> Edit Feature Settings -> "Detailed Errors"
Also note that if the content of the error page sent back is quite short and you're using IE, IE will happily ignore the useful content sent back by the server and show you its own generic error page instead. You can turn this off in IE's options, or use a different browser.
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