adds context and skippedstring to CSS messages

Although it does not appear on the documentation, the validation response for CSS includes the `context` and `skippedstring` properties, like this:

	<m:error>
		<m:line>2</m:line>
		<m:errortype>parse-error</m:errortype>
		<m:context> audio, canvas, video </m:context>        
		<m:errorsubtype>
		    unrecognized
		</m:errorsubtype>
		<m:skippedstring>
		    ;*display:inline;
		</m:skippedstring>

		<m:message>
                Error de análisis sintáctico
		</m:message>
	</m:error>

This commit adds the ability to parse these properties from the response.