input:

add to config/custom-processors.xml:
  <processor name='oxf:/load-try'>
    <class name='org.mathdox.loadtryprocessor.LoadTryProcessor'/>
  </processor>

example input:
<p:processor name='oxf:/load-try'>
  <p:input name='config'>
    <config>
      <resource>oxf:/style.css</resource>
    </config>
  </p:input>
  <p:output name='data'/>
  <p:output name='result'>
</p:processor>

1) FILE (style.css) exists
- result:
  <wrapper>
    <result>success</result>
  </wrapper>
- data: contents of the file

2) FILE (style.css) does not exist
- result:
  <wrapper>
    <result>fail</result>
  </wrapper>
- data: 
  <error>
    <stacktrace>
      <!-- java stacktrace -->
    </stacktrace>
  </error>

example input that handles an XML file and generates a SAX output:
<p:processor name='oxf:/load-try'>
  <p:input name='config'>
    <config>
      <resource>oxf:/footer.xhtml</resource>
      <mode>xml</mode> <!-- NOTE: added mode xml -->
    </config>
  </p:input>
  <p:output name='data'/>
  <p:output name='result'>
</p:processor>

Note that a validation error might not be caught. If the file exists and SAX
initialization can be done, the result success will be passed.

Maybe in the future validating beforehand might also be an option. Note that
this might be slower for large files, since the file first has to be validated
completely before the start of the file can be send (which in effect means it
has to be parsed twice).