Fixed WMS response if remote layer cannot be retrieved

[dstenger]

Fixes #1926

Currently, when a remote layer is requested and this layer does not respond with an image (e.g. exception, timeout), the WMS response of deegree is an empty image. Thus, the user does not know if the layer does not work or if there is no data in the requested BBOX.

This PR fixes the behavior by recognizing the erroneous response of the remote layer and passing an error message to the user of the deegree WMS.

Examples for new behavior:

• WMS response if remote layer cannot be requested:

<?xml version='1.0' encoding='UTF-8'?>
<ServiceExceptionReport xmlns="http://www.opengis.net/ogc">
  <ServiceException code="NoApplicableCode">Error retrieving remote map.</ServiceException>
</ServiceExceptionReport>

• Further details can be found in the logging of deegree.
• WMS response if remote layer cannot be requested and EXCEPTIONS=XML is passed: Same behavior as before.
• WMS response if remote layer cannot be requested and EXCEPTIONS=INIMAGE is passed: Image is returned with this text in upper left corner: Error: Error retrieving remote map.
• If the layer is a sub layer and the group layer is requested: The behavior is exactly the same as when requesting the sub layer directly (exceptions are returned).