Easy NPE in Browse Version

Browsing for non-existent versions within a project produces inconsistent results. Most commonly it will throw a NPE.

http://jira.atlassian.com/browse/JRA/fixforversion/1
boom!

it will also 500 with any number of digits up to:
http://jira.atlassian.com/browse/JRA/fixforversion/11111111111111111111
which will simply go to the Browse Project screen (same as if there is no fix version specified).

We need to handle this properly. VersionContextImpl should not be able to be built with a null version, and the validation in the action should return a nice error page.

java.lang.NullPointerException
	at com.atlassian.jira.plugin.versionpanel.VersionContextImpl.getProject(VersionContextImpl.java:43)
	at com.atlassian.jira.plugin.webfragment.model.VersionHelper.(VersionHelper.java:17)
	at _jsp._secure._views._browser._browseversion__jsp._jspService(_browseversion__jsp.java:272)
	at com.caucho.jsp.JavaPage.service(JavaPage.java:60)
        ...