Batch API needs to return useful error messages

This came up while testing #363262: Missing index on url_alias table, but works for really any Batch API-related thing. When Batch API encounters an error, instead of something actually helpful such as "I'm sorry, that index already exists," it gives you the completely unhelpful:

An unrecoverable error has occurred. You can find the error message below. It is advised to copy it to the clipboard for reference.
Please continue to the error page
An HTTP error 500 occurred.
http://localhost/core/update.php?id=7&op=do

An easy way to replicate this is to rename system_update_7018 it to system_update_7999 and run it again.

This is getting masked right now because we removed the ability to select past updates from update.php. It also happens in SimpleTest from time to time. Needs to be fixed before release or the support requests will be unbearable.