Add error handling to _connect function in PureISCSIDriver

The driver was throwing an exception when trying to connect a volume
to a host when they were already connected.  This change catches the
exception and looks up the existing connection information to use as
a return value.

Change-Id: Id0e6b1d3c3cd1e51745e97f99eef88a15e285526
Closes-Bug:  1403631
Merge "Add error handling to _connect function in PureISCSIDriver"