Pass along the original exception when an ssh connection fails

Otherwise, the stack trace doesn't really tell anything.
See for instance [1].

[1] https://www.eclipse.org/forums/index.php/t/1088535/

Change-Id: If22f2c63c36fec6b32818d2c2acecf20531b4185
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

org.eclipse.jgit/src/org/eclipse/jgit/transport/JschConfigSessionFactory.java

@@ -153,10 +153,13 @@ public abstract class JschConfigSessionFactory extends SshSessionFactory {
 
 		} catch (JSchException je) {
 			final Throwable c = je.getCause();
-			if (c instanceof UnknownHostException)
-				throw new TransportException(uri, JGitText.get().unknownHost);
-			if (c instanceof ConnectException)
-				throw new TransportException(uri, c.getMessage());
+			if (c instanceof UnknownHostException) {
+				throw new TransportException(uri, JGitText.get().unknownHost,
+						je);
+			}
+			if (c instanceof ConnectException) {
+				throw new TransportException(uri, c.getMessage(), je);
+			}
 			throw new TransportException(uri, je.getMessage(), je);
 		}