Added log when currency rates import fail

[kyrena]

Description

When currency cron "Scheduled Import Settings" is enabled, in System > Configuration > Currency Setup, when there is an import error, errors are not logged.

With this PR, error is logged in exception.log (Mage::logException), cron job is marked as "error" ($cron->setIsError), and cron message contains the error log.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
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[sreichel]

Pls dont change signatures of used methods. Better implement new ones.

[kiatng]

How about this?

if (func_num_args() && func_get_args()[0] instanceof Mage_Cron_Model_Schedule) {
    $schedule = func_get_args()[0];
    //....
}

[sreichel]

I'd add a new method w/ parameter and all logic in and forward to this method. Similiar i did with #2940.

[kiatng]

For BC, it's tricky to remove entries in the translation file Mage_Directory.csv.

[fballiano]

Since the original author didn't answer any of the notes I'll close this PR.
@kyrena feel free to reopen if you're willing to fix it :-)