webadmin, core: Return specific provider authentication error

On addProvider form the user fills authentication parameters to
communicate with external provider.
This patch adds more detailed warning message if the parameters are
wrong and the authentication failed.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1113000
Change-Id: Ia8d6540b115454cfd2aa055ba711694f74855164
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>