bug fix: Only check for ISO images in active pools

The wrong exception was expected while searching for ISO files in the storage
pools.
Independent of the exception raised it should not block users to create
a template, so use the generic Exception to catch any error.

Fix it to avoit the error below.

Request Headers:
  COOKIE: kimchi=a50f5cb488c331754db1a642eaba89f7fe71c0b8;
userid=alinefm
  HOST: localhost:8000
  CONNECTION: keep-alive
  Remote-Addr: 127.0.0.1
  ACCEPT: application/json, text/javascript, */*; q=0.01
  USER-AGENT: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:27.0)
Gecko/20100101 Firefox/27.0
  X-REQUESTED-WITH: XMLHttpRequest
  ACCEPT-LANGUAGE: en-us,en;q=0.7,pt-br;q=0.3
  Content-Type: application/json
  REFERER: http://localhost:8000/
  ACCEPT-ENCODING: gzip, deflate
[28/Feb/2014:12:27:50] HTTP Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cprequest.py", line 656, in respond
    response.body = self.handler()
  File "/usr/lib/python2.7/dist-packages/cherrypy/lib/encoding.py", line 188, in __call__
    self.body = self.oldhandler(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/cherrypy/_cpdispatch.py", line 34, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "/home/alinefm/kimchi/src/kimchi/control/base.py", line 274, in index
    return self.get(filter_params)
  File "/home/alinefm/kimchi/src/kimchi/control/storagevolumes.py", line 71, in get
    res_list = get_list(*self.model_args)
  File "/home/alinefm/kimchi/src/kimchi/model/storagevolumes.py", line 170, in get_list
    pool.refresh(0)
  File "/home/alinefm/kimchi/src/kimchi/model/libvirtconnection.py", line 62, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/libvirt.py", line 2220, in refresh
    if ret == -1: raise libvirtError ('virStoragePoolRefresh() failed', pool=self)
libvirtError: Requested operation is not valid: storage pool is not active

Signed-off-by: Aline Manera <alinefm@br.ibm.com>