plpython and error catching

What is the expected behavior of a construct like this:

def insert_xml(elem):
id=int(elem.findtext('PMID'))
try:
plpy.execute(plan,[unicode(ET.tostring(elem)),id])
except:
plpy.execute(plan2,[unicode(ET.tostring(elem)),id])

id is a primary key on the table into which I am inserting. "plan" is
the execute plan for inserting new data. "plan2" is for updating data
already in the database. When I run this, I am not able to catch
errors of this type:

WARNING: plpython: in function insert_medline_file:
DETAIL: plpy.Error: Unknown error in PLy_spi_execute_plan
ERROR: duplicate key value violates unique constraint "medlinexml_pkey"
CONTEXT: SQL statement "insert into medlinexml(content,id) values
(xmlparse(CONTENT $1),$2)"

Why am I not able to catch this error and execute the update plan?
The manual (8.3beta2) implies that errors generated in functions are
genuine python errors that I should be able to catch.

Thanks,
Sean