pyodbc does not throw on SQL Server error

I am trying to use pyodbc (with Python 2.7) to call a stored procedure to insert records into a SQL Server 2012 table. I am passing a temporary table.

I dumped out my sql and when executed through the SQL Server Management console, it generated the following Foreign Key error:

Msg 547, Level 16, State 0, Procedure spInsertBondTickerValues, Line 26
The INSERT statement conflicted with the FOREIGN KEY constraint "FK__BondTickerValue__756D6ECB".
The conflict occurred in database "QuantDev", table "dbo.Tickers".
The statement has been terminated.

However, pyodbc did not raise an exception. How would I test the resulting cursor or connection to know that a problem occurred, and how do I get the error message?

Thank you very much.

EDIT Here is the full sql text:

DECLARE @rawTbl [dbo].TickerValueTableType
INSERT INTO @rawTbl (Ticker, BBName, LastValue, ValueTime, SourceDescr) VALUES
('IBM', 'Equity', 179.230000, '2013-11-01 00:00:00.000000', 'Bloomberg'),
('SPX', 'Index', 1803.710000, '2013-12-10 00:00:00.000000', 'Bloomberg')
EXEC [dbo].spInsertBondTickerValues @rawTbl

EDIT 2 Here is the relevant Python code:

def execSQLwithCommit(self, sql):
    cursor = self.conn.cursor()
    cursor.execute(sql)
    self.conn.commit()

where the connection has been previously made via

self.conn = pyodbc.connect(app      = appName,
                           driver   = '{SQL Server Native client 11.0}',
                           server   = server,
                           database = db,
                           Trusted_Connection = 'yes')