Fix error detection when GetCapabilities is an ExceptionReport

This syncs the behavior with that of DescribeSensor requests (see https://github.com/asascience-open/OWSLib/blob/dd2b6c643c9f17e869623079740e3a0f098e38fe/owslib/swe/observation/sos100.py#L119-L120)

When a GetCapabilities request returned an ExceptionReport, it wasn't properly being detected and would result in an AttributeError down the line.