Fix exception log msg in attach volume method

Attach volume happened when volume's status is attaching or available, but
the exception msg only contain info about "available". So fix it.

Change-Id: I1caf83b94b3e4158eff88871a4187041a25e5d1c
Closes-Bug: #1270020
Merge "Fix exception log msg in attach volume method"
(Test commit) Change README to reflect Jio's fork of Cinder

Change-Id: I3f26c0a736e63725aec24ca2fa12c707b8e6460f