Add a second layer of error checks to the identifier

[roomthily]

This is likely to be an invalid response but not one with a known error structure.

If it's an OGC service and it's a GetCapabilities response and it contains a Capability/Request method element for a known secondary data access point (DescribeCoverage, GetCoverage, GetMap, GetFeature, etc) and it DOESN'T have a coverage/feature/layer definition, mark is_error as True and it is unparsable. (It isn't unparsable really, but maybe we should consider those too invalid to proceed?)

[roomthily]

See also: http://geobrain.laits.gmu.edu/cgi-bin/wcs-all?service=wcs&version=1.1.0&request=getcapabilities&sections=ServiceProvider

where the response is limited to the ServiceProvider section based on the sections query parameter.