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 rasing 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.