better error message when using formatSealed on case objects but missing imports

[cvogt]

needs these imports, but doesn't say so:

import SingletonEncoder.simpleName // required for formatSingleton import org.cvogt.play.json.implicits.formatSingleton // required if trait has object children

[cvogt]

also should probably be import org.cvogt.play.json.SingletonEncoder.simpleName // required for formatSingleton import org.cvogt.play.json.implicits.formatSingleton // required if trait has object children

here and in docs