Provide more detail for exception "Invalid ref name: HEAD"

Current, when a JGit internal exception "Invalid ref name: HEAD" occurred, we don't have any valuable information about the problem:
 

Caused by: org.eclipse.jgit.api.errors.JGitInternalException: Invalid ref name: HEAD
	at org.eclipse.jgit.api.ResetCommand.call(ResetCommand.java:170)
	at com.nuxeo.studio.core.versioning.VersioningAbstract.resetOnHead(VersioningAbstract.java:456)
	... 99 more

We need to have more detail for understanding:

• What is the value of the Git reference, e.g. is it refs/heads/master?
• Does the reference exist?
• Does the reference point to an existing object id?