API Middleware fault: Log exception type

API Middleware fault: Log exception type

This causes a fault such as the following to log the type:

ERROR cinder.api.middleware.fault [req-7d2... ddf... 658...]
  Caught error: <class 'oslo_db.exception.DBDataError'> (pymysql.err.InternalError)
  (1366, u"Incorrect string value: '\\xF0\\x9F\\x8C\\x8B' for column 'display_name'
   at row 1") [SQL: u'INSERT...

Without this info (DBDataError), it is rather difficult to
determine which exception actually occurred.

Change-Id: I51ca64847b7d82c4258e134161e1b23f232004e5