lib: handle both HTML, unsafe strings, and exceptions passed to helpers.flash()

Before, h.flash would trust any input to contain html ... and callers would
convert exceptions to string, often with a simple str() or unicode() ... which
really didn't deserve to be trusted.

Instead, only trust messages that have a __html__ and escape anything else ...
but also apply str/unicode on the parameter so the caller doesn't have to but
*can* pass an exception directly.