Improved error message for invalid header cell in an Excel sheet.
Fixed off by one in the row number reported.
create jaxb context only once (story #1962)
Merge pull request #664 from dennishendriksen/master

Create JAXBContext only once (story #1962)
Merge pull request #662 from djvanenckevort/master

Improved reporting of errors in uploaded OMX excel sheet.