Null pointer exception when saving to a file without an absolute path

[daust]

See: https://gitq.com/daust/JasperReportsIntegration/topics/62/saving-jasper-report-output/3

When saving a file to the server, an absolute filename is required per documentation. Yet, when a relative path is given, it will simply cause a null pointer exception. Here, a better error message could be provided.