Improve error message regarding mismatched tmp directories when sql-syncing between systems with different fs layout

I executed "drush sql-sync @live @dev" where "@live" is a linux system and "@dev" is a Mac OS notebook.

# drush -v -y sql-sync @live @dev
# ...
Calling system(ssh  web1@live 'mysqldump --result-file /private/tmp/live_usr_web1_1.sql.eCh8Bj --single-transaction --opt -Q  usr_web1_1 --host=localhost --user=web1 --password=XXXXX ');

The error message is

mysqldump: Can't create/write to file '/private/tmp/vserver.rueweb.de_usr_web1_1.sql.TjyIrx' (Errcode: 2)

The drush command on the Mac client uses its own settings to specify the temp folder on the source machine. But that's a linux machine that doesn't have a "/private" folder.

My workaround was to create the folder "/private/tmp" on the source machine.

PS: The error message should mention that there was an error on the source machine, just to ease the trouble shooting.