templater: explode if we try to emit a str
Without this if branch, we infinitely recurse in _flatten, which is
very confusing. Something in an hgweb template is trying to write out
a string instead of a bytes on Python 3, and this at least makes it
crash politely.
Differential Revision: https://phab.mercurial-scm.org/D1088
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed