ENH: more graceful handling of invalid IOobject headers (issue #539)

- With special-purpose templating it is possible to have file contents
  that almost look like an OpenFOAM file, but which are not.
  The contents do not need to be deliberately tricky, even the simplest
  README:

      FoamFile is the first word parsed in OpenFOAM files

  will trigger problems.
  We now trap any IOerror on these and reject this type of file as invalid.