ENH: error handling for empty surfaces in surfaceFieldValue (#2966)

- for workflows with appearing/disappearing patches (for example)
  can specify that empty surfaces should be ignored or warned about
  instead of raising a FatalError.

  Note that this handling is additional to the regular top-level
  "errors" specification. So specifying 'strict' will only actually
  result in a FatalError if the "errors" does not trap errors.

- "ignore" : any empty surfaces are simply ignored and no
  file output (besides the header).

- "warn" : empty surfaces are warned about a few times (10)
  and the file output contains a NaN entry

- "strict" : corresponds to the default behaviour.
  Throws a FatalError if the surface is empty.
  This error may still be caught by the top-level "errors" handling.