(tal) start tags with missing end tag are silently ignored

[d-maurer]

Reproduced by (e.g.)

from chameleon.zpt.template import PageTemplate
ts = """<tal:define define="x 1"><span tal:content="x"/>"""
t = PageTemplate(ts)
t()

It is quite frequent to forget end tags (or place them at the wrong place in the template structure). Silently ignoring the corresponding start tags makes the analysis of such errors much more difficult.