(mh-insert-letter): If a message number isn't given, throw an error

(mh-insert-letter): If a message number isn't given, throw an error
rather than using a potentially incorrect message number (closes SF
#1473729). In addition, use the cur message if mh-sent-from-msg is
nil (when sending a message, in contrast to replying). Move conversion
of int to string into interactive stanza so body can assume variables
are of proper type.