#48 Graphs - headless option not set - die gracefully

[Mike King]

If CentraView is installed on a server without a graphical interface
(Xwindows or MS Windows), *and* the java.awt.headless option is not
set to true on the VM, the graphs will not be able to be generated.

This is a known issue with the JGraph library. It's easily fixed by adding:

-Djava.awt.headless=true

to the JAVA_OPTS variable. However, if the installed fails to do so
properly, we should still handle the error gracefully, instead of showing
corrupt images and throwing exceptions to the log.

Somehow, check to see if we have the appropriate configuration, If not,
serve a static image with instructions to the end user.