* VuFindSearch/Backend/Exception/BackendException.php (static): Prefix
  reason phrase with status code.

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

* 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.

Resolves VUFIND-659.

* ../tests/unit-tests/src/Search/Solr/MultiIndexListenerTest.php
  (setup): Create Connector with dummy handler map.

* VuFindSearch/Backend/Solr/HandlerMap.php (setHandlerMap):
  Correctly set fallback handler.

Also adjusted continuous integration setup accordingly.

* VuFindSearch/Backend/AbstractHandlerMap.php (apply): New
  function. Apply defaults, appends, invariants.
  (prepare): Factor out d/a/i application, undeclare final.

* VuFind/Search/Factory/AbstractSolrBackendFactory.php
  (createConnector): Use handler map.

* VuFindSearch/Backend/Solr/HandlerMap.php (__construct): Make map an
  optional argument.

* VuFindSearch/Backend/Solr/Connector.php (getQueryInvariants)
  (getQueryDefaults, getQueryAppends, setQueryInvariants)
  (setQueryDefaults, setQueryAppends, addQueryInvariants)
  (addQueryDefaults, addQueryAppends, $invariants, $defaults)
  ($appends): Removed.
  (getMap): New function. Return handler map.
  (retrieve, similar, search, terms): Use handler map to prepare
  params and select handler.
  (select, prepare): Removed.

* VuFindSearch/Backend/Solr/HandlerMap.php (getParameters)
  (setParameters): Declare public, rename parameter $key to $type.
  (addParameter): New function. Add parameters.

Get jetty to use system temporary folder when extracting war file rather than creating a directory in the Vufind root - comment out tempDirectory setting in /solr/jetty/contexts/solr.xml

* VuFindSearch/Backend/Solr/HandlerMap.php: New class. SOLR
  handler map.

* VuFindSearch/Backend/AbstractHandlerMap.php: Expect ParamBag as
  return value for D/A/I.

* VuFindSearch/ParamBag.php (params): Removed. Use getArrayCopy()
  instead.

* VuFindSearch/ParamBag.php (exchangeArray): Use internal function
  to set new parameter array.

* VuFindSearch/Backend/AbstractHandlerMap.php: New class. Abstract
  base class of handler maps.

* VuFindSearch/ParamBag.php (exchangeArray): New function. Exchange
  parameter array.

* VuFindSearch/Backend/Solr/Connector.php (MAX_GET_URL_LENGTH): Fix
docstring.

Expanded test suite to match.

Expanded test suite for writer.

Thanks to Arek Danszczyk and Christian Dabrowski.

* VuFindSearch/Backend/Exception/BackendException.php ($tags): New
  property. Tags.
  (addTag, getTags, hasTag): New functions. Tagging support.

* VuFindSearch/Service.php (search, retrieve,
  retrieveBatch) (similar): Catch BackendException, trigger error
  event & rethrow.