#238 Exceptions in NeXML export

[Mark Holder]

For several studies, requesting NeXML results in a stacktrace. See for example:
http://treebase.org/treebase-web/search/downloadAStudy.html?id=1001&format=nexml
In some cases these pages load as empty pages. The list of IDs is S1001 S10071 S10316 S10503 S11762 S11811 S12156 S1224 S12742 S12891 S13351 S13572 S1360 S1936 S1967 S1984 S202 S2377 S377 S400 S412 S613 S852 S981