verification of valid server

[Reimar Bauer]

Currently any Url which exists but is not a WMS server gives a bad failure status.

ERROR: We cannot load the capability document!

<type 'exceptions.AttributeError'>
'NoneType' object has no attribute 'find'

ERROR: cannot load capabilities document.. no layers can be used in this view.

The WMSCapabilitiesReader has to verify that it is be used on a proper server or reject access by a correct error message.

[Reimar Bauer]

ERROR: We cannot load the capability document!

<class 'requests.exceptions.HTTPError'>

404 Client Error: Not Found for url: nnn ?service=WMS&request=GetCapabilities&version=1.1.1

[Reimar Bauer]

the error message was improved