implements switch to abort a migration on an error

[bjoernHeneka]

I'm using the propel migration within an automated rollout process. Problem is that if within a migration file are more than 1 statement,

SET FOREIGN_KEY_CHECKS = 0; 
SQL with error
SET FOREIGN_KEY_CHECKS = 1;

The phing tasks continues migrating. With the change i pushed you can set a configuration flag

propel.migration.abortOnError = false|true

if set to true the migration aborts with this task. If false it behaves as now.