Exceptions during validation hide follow-on issues

[prettybits]

When an exception occurs during validation, e.g. due to a schema validation error this is currently reported as an issue for CSIP0 and further validation seems to be skipped and requirements report as passed.

E.g. I got a schema validation error due to faulty IDREFs:

{
    "specification" : "CSIP-2.0.4",
    "id" : "CSIP0",
    "name" : "METS Schema",
    "location" : "",
    "description" : "Validation of METS Schema",
    "cardinality" : "",
    "level" : "MUST",
    "testing" : {
      "outcome" : "FAILED",
      "issues" : [ "[SAXParseException] cvc-id.1: There is no ID/IDREF binding for IDREF 'DMDLOG_0001'. (file: /home/prettybits/SIPs/CSIP0/METS.xml, line: 46, column: 13)" ],
      "warnings" : [ ],
      "notes" : [ ]
    }
  }

and a passed check for CSIP23:

{
    "specification" : "CSIP-2.0.4",
    "id" : "CSIP23",
    "name" : "Type of link",
    "location" : "mets/dmdSec/mdRef[@xlink:type=’simple’]",
    "description" : "Attribute used with the value “simple”. Value list is maintained by the xlink standard.",
    "cardinality" : "1..1",
    "level" : "MUST",
    "testing" : {
      "outcome" : "PASSED",
      "issues" : [ ],
      "warnings" : [ ],
      "notes" : [ ]
    }
  }

even though that actually failed after fixing the reference issue with the errors reported in #162.