ENH: finer granularity for handling functionObject failure (#1779)

- additional "errors" entry with enumerated values
  (default|warn|ignore|strict) for defining warning or error at
  construct or runtime stage

- default : construct = warn, runtime = fatal
- warn    : construct = warn, runtime = warn
- ignore  : construct = silent, runtime = silent
- strict  : construct = fatal, runtime = fatal

The errors control can be added at the top-level and/or for individual
function objects.