Backport to 7.9: Console stops working when hst reference property starts with a slash

Some hst properties reference other hst nodes. The link to that node is shown as a link after the property. When such a property starts with a forward slash, the HstReferenceEditor throws an error and that node cannot be edited anymore.

Before trying to process the value, the Editor should check the value. If it starts with a forward slash, give an apropriate message to the user about that.

This is the stacktrace that appears in the tomcat log:
[INFO] [talledLocalContainer] Caused by: java.lang.IllegalArgumentException: relPath is not a relative path: {}

{http://www.hippoecm.org/hst/nt/2.1}

hst

{http://www.hippoecm.org/hst/nt/2.1}

configurations {}myhippoproject

{http://www.hippoecm.org/hst/nt/2.1}

templates {}twocolumns-right
[INFO] [talledLocalContainer] at org.apache.jackrabbit.spi.commons.name.PathFactoryImpl.create(PathFactoryImpl.java:47)
[INFO] [talledLocalContainer] at org.apache.jackrabbit.core.NodeImpl.getNodeId(NodeImpl.java:271)
[INFO] [talledLocalContainer] at org.apache.jackrabbit.core.NodeImpl.resolveRelativeNodePath(NodeImpl.java:224)
[INFO] [talledLocalContainer] at org.apache.jackrabbit.core.NodeImpl.hasNode(NodeImpl.java:2281)
[INFO] [talledLocalContainer] at org.hippoecm.repository.decorating.NodeDecorator.hasNode(NodeDecorator.java:300)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.editor.HstReferenceEditor$ReferenceLink.getConfigurationNode(HstReferenceEditor.java:241)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.editor.HstReferenceEditor$ReferenceLink.getHstReferencedNode(HstReferenceEditor.java:195)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.editor.HstReferenceEditor$ReferenceLink.load(HstReferenceEditor.java:109)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.editor.HstReferenceEditor$ReferenceLink.<init>(HstReferenceEditor.java:92)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.editor.HstReferenceEditor.<init>(HstReferenceEditor.java:65)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.editor.HstReferenceEditorPlugin.createEditor(HstReferenceEditorPlugin.java:68)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.editor.PropertyValueEditor.createValueEditor(PropertyValueEditor.java:144)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.console.editor.PropertyValueEditor.populateItem(PropertyValueEditor.java:75)