Catch <values> without <value> error.

This catches responses that return <values> without associated <value>.

Example Below: 
  
  url = "http://waterservices.usgs.gov/nwis/iv/?format=waterml,1.1&huc=02&startDT=2014-10-11T00:00:00.000&endDT=2014-10-11T00:30:00.000&parameterCd=00060,00065"
  data <- getWaterML1Data(url)

WaterML Response (chunk)
  <ns1:timeSeries xmlns:ns1="http://www.cuahsi.org/waterML/1.1/" name="USGS:01350080:00065:00011">
  <ns1:sourceInfo xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="ns1:SiteInfoType">
  <ns1:siteName>MANOR KILL AT WEST CONESVILLE NEAR GILBOA NY</ns1:siteName>
  <ns1:siteCode network="NWIS" agencyCode="USGS">01350080</ns1:siteCode>
  <ns1:timeZoneInfo siteUsesDaylightSavingsTime="false">
  <ns1:defaultTimeZone zoneOffset="-05:00" zoneAbbreviation="EST"/>
  <ns1:daylightSavingsTimeZone zoneOffset="-04:00" zoneAbbreviation="EDT"/>
  </ns1:timeZoneInfo>
  <ns1:geoLocation>
  <ns1:geogLocation xsi:type="ns1:LatLonPointType" srs="EPSG:4326">
  <ns1:latitude>42.37694444</ns1:latitude>
  <ns1:longitude>-74.4130556</ns1:longitude>
  </ns1:geogLocation>
  </ns1:geoLocation>
  <ns1:siteProperty name="siteTypeCd">ST</ns1:siteProperty>
  <ns1:siteProperty name="hucCd">02020005</ns1:siteProperty>
  <ns1:siteProperty name="stateCd">36</ns1:siteProperty>
  <ns1:siteProperty name="countyCd">36095</ns1:siteProperty>
  </ns1:sourceInfo>
  <ns1:variable ns1:oid="45807202">
  <ns1:variableCode network="NWIS" vocabulary="NWIS:UnitValues" default="true" variableID="45807202">00065</ns1:variableCode>
  <ns1:variableName>Gage height, ft</ns1:variableName>
  <ns1:variableDescription>Gage height, feet</ns1:variableDescription>
  <ns1:valueType>Derived Value</ns1:valueType>
  <ns1:unit>
  <ns1:unitCode>ft</ns1:unitCode>
  </ns1:unit>
  <ns1:options>
  <ns1:option name="Statistic" optionCode="00011"/>
  </ns1:options>
  <ns1:noDataValue>-999999.0</ns1:noDataValue>
  </ns1:variable>
  <ns1:values>
  <ns1:method methodID="1">
  <ns1:methodDescription>[backup from well]</ns1:methodDescription>
  </ns1:method>
  </ns1:values>
  <ns1:values>
  <ns1:value qualifiers="P" dateTime="2014-10-11T00:00:00.000-05:00">0.85</ns1:value>
  <ns1:value qualifiers="P" dateTime="2014-10-11T00:15:00.000-05:00">0.85</ns1:value>
  <ns1:value qualifiers="P" dateTime="2014-10-11T00:30:00.000-05:00">0.85</ns1:value>
  <ns1:qualifier qualifierID="0" ns1:network="NWIS" ns1:vocabulary="uv_rmk_cd">
  <ns1:qualifierCode>P</ns1:qualifierCode>
  <ns1:qualifierDescription>Provisional data subject to revision.</ns1:qualifierDescription>
  </ns1:qualifier>
  <ns1:method methodID="12">
  <ns1:methodDescription/>
  </ns1:method>
  </ns1:values>
  </ns1:timeSeries>