If a plugin descriptor is missing the 'key' attribute a NullPointerException is thrown.

Need to display a better error message.

The exception looks like this:
java.lang.NullPointerException
at com.atlassian.plugin.loaders.SinglePluginLoader.loadPlugins(SinglePluginLoader.java:52)
at com.atlassian.plugin.loaders.SinglePluginLoader.getPlugins(SinglePluginLoader.java:133)
at com.atlassian.plugin.DefaultPluginManager.init(DefaultPluginManager.java:31)
at com.atlassian.jira.plugin.JiraPluginManager.start(JiraPluginManager.java:35)
at com.atlassian.jira.ComponentManager.quickStart(ComponentManager.java:213)
at com.atlassian.jira.ComponentManager.start(ComponentManager.java:194)
at com.atlassian.jira.upgrade.ConsistencyLauncher.contextInitialized(ConsistencyLauncher.java:25)
at com.caucho.server.http.Application.init(Application.java:1843)
at com.caucho.server.http.VirtualHost.init(VirtualHost.java:728)
at com.caucho.server.http.ServletServer.initHosts(ServletServer.java:874)
at com.caucho.server.http.ServletServer.initInternal(ServletServer.java:719)
at com.caucho.server.http.ServletServer.init(ServletServer.java:530)
at com.caucho.server.http.ResinServer.init(ResinServer.java:393)
at com.caucho.server.http.ResinServer.main(ResinServer.java:1154)
at com.caucho.server.http.HttpServer.main(HttpServer.java:103)