Hyrax error messages when a DMR++ references a file/url that does not exist are not useful

[jgallagher59701]

We found this while working on #1332. We changed the filename from ...HDF5 to 3B-HHR.MS.MRG.3IMERG.19980101-S000000-E002959.0000.V07B.HDF5.DISABLED. When we tried to access data using the DMR++, we got:

<?xml version="1.0" encoding="UTF-8"?>
<dap4:Error xmlns:dap4="http://xml.opendap.org/ns/DAP/4.0#" httpcode="500">
  <dap4:Message>ERROR - Unable to parse expected &amp;lt;BESError&amp;gt; object from stream! Message: Error on line 102: Content is not allowed in trailing section.</dap4:Message>
  <dap4:Context />
  <dap4:OtherInformation />
</dap4:Error>

First, fix the text. We should be able to tell the user that there's no underlying data available. Make sure to sort out the case where there is just one URI at teh top of the DMR++ and the case where there are several URIs.

Second, the <BESError> is just gross. There's no reason to preserve XML syntax at this point. Make that message

<dap4:Message>ERROR - Unable to parse expected BESError object from stream! Message: Error on line 102: Content is not allowed in trailing section.</dap4:Message>