Store server response in backend exception

* VuFindSearch/Backend/Solr/Connector.php (send): Pass response to
  exception constructor.
* VuFindSearch/Backend/Exception/BackendException.php (__construct):
  Make original response mandatory constructor argument.
  (getResponse): New function. Return original server response.
Implement and use factory for BackendException

* VuFindSearch/Backend/Solr/Connector.php (send): Use exception
  factory.
* VuFindSearch/Backend/Exception/BackendException.php
  (createFromResponse): New function. Create exception based on
  server response.
Prefix reason phrase with status code

* VuFindSearch/Backend/Exception/BackendException.php (static): Prefix
  reason phrase with status code.
Use BackendException factory in SRU/Connector, throw BackendException

* VuFindSearch/Backend/SRU/Connector.php (process): Throw
  BackendException instead of generic Exception.
  (checkForHttpError): Use BackendException factory.
Implement SOLR 3.x error listener