webadmin: Display correct error when adding new storage

When adding a new Storage Domain (of NFS/Local type) the error displayed
when the connection was failed was always "Error creating a storage domain"
instead of the actual error from VDSM (no permission error etc.), the connect
command was changed to propegated the VDSM error to the client there it will
be tranlsated and be displayed as the translated user friendly value

Change-Id: Ib8f721d3f10596253ca194a36139ba9ebcdff246
Bug-Url: https://bugzilla.redhat.com/843440
Signed-off-by: Tal Nisan <tnisan@redhat.com>