Error message in catch lacks useful information for p:load

[dwcramer]

See also issue #303.

I'm using p:load to load a doc and wrap it in a p:try/p:catch in XProc (Calabash 1.22). When the document is malformed or unparsable, I get very little information about the nature of the problem in the catch:

<c:errors xmlns:c="http://www.w3.org/ns/xproc-step">
      <c:error xmlns:err="http://www.w3.org/ns/xproc-error" code="err:XC0011"
          href="file:/path/to/pdf.xpl"
          line="6" column="65">Could not load
          /path/to/Quinoa_Axe_Throwing_Specification.xml
          (file:/path/to/pdf.xpl)
          dtd-validate=false</c:error>
 </c:errors>

It would be super useful if I could tell them whether the parsing of the file failed because of an unresolved entity or if it's instead an unclosed tag. The parser certainly knows that--it shows up in the "Underlying exception" in the stack trace:

lineNumber: 9; columnNumber: 55; The entity "foo" was referenced, but not declared.

Couldn't Calabash parser put that stack trace in a c:error and let me deal with it? After all, I'm using a try/catch because I want to catch exceptions. I can then present that stack trace to the user with all its useful information.