ItemHolder->import() does not report error state

[limolitz]

When Magmi_ProductImportEngine->processDataSourceLine() called by ItemHolder->import() fails and catches the Exception, it rolls back the transaction, logs the Exception and the error state is stored in $res["ok"]. This error state is propagated to Magmi_ProductImport_DataPump->ingest(), but then the error state is thrown away in ItemHolder->inject() and never reaches the caller.

This makes it impossible for the calling code to check if there was an error in the execution (which, in our case, is raised by #479.)