1097790 - check task details of erratum upload to determine if task succeeded

Previously, failing erratum uploads were being shown as succeeded in
pulp-admin.

This commit checks for task.details.error, and if it is populated, assumes that
the task failed.

NOTE: this is very similar to 1fef7b67, which was bcourt's fix for rpm upload
error checking.
Merge pull request #507 from beav/erratum-fail

1097790 - check task details of erratum upload to determine if task succeeded