IssueDocument should be hardened against ill-behaved CF plugins

As discovered while troubleshooting reindex problems on an internal JIRA instance, a runtime exception raised by an issue's custom field prevents that issue from being indexed at all. IssueDocument.getDocument(Issue) should be hardened such that only the problem custom field is dropped from the indexing, not the entire issue. The data will still be wrong, but partial data is better than no data.

Ideally, the exception should be remembered and re-thrown after the issue has been indexed so that we can still report the problem.