fix the error log print in encryptor __init__.py

when attach an encrypted volume to vm, the error
log print is not right in encryptor __init__.py.
It will raise wrong exception info.

This change will fix the log print using correct
format.

Change-Id: Ibbe41e64ae6320efa53bdd97cdd5312457e65be7
Closes-bug: #1384531
Merge "fix the error log print in encryptor __init__.py"