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
Avoid unset warnings.
Throw BackendException on SRU XSLT processing error

* VuFindSearch/Backend/SRU/Connector.php (process): Throw
  BackendException on XSLT processing error, don't check for XML
  prolog.
Updated dependencies.
Implement HttpErrorException for HTTP error exceptions

* VuFindSearch/Backend/SRU/Connector.php (checkForHttpError): Dto.
* VuFindSearch/Backend/Solr/Connector.php (send): Use
  HttpErrorException instead of BackendException.
* VuFindSearch/Backend/Exception/HttpErrorException.php: New
  class. Base class of HTTP error exceptions.
* VuFindSearch/Backend/Exception/HttpErrorException.php
  ($response, __construct, getResponse, createFromResponse): Moved
  to HttpErrorException.
Simplified Summon code to use native functionality.
Check for HttpErrorException instead of BackendException

* VuFind/Search/Solr/V3/ErrorListener.php: Check for
  HttpErrorException instead of BackendException.
Merge branch 'master' of ssh://vufind.git.sourceforge.net/gitroot/vufind/vufind