Better error detection in msgtoqwk()

Have msgtoqwk() return a negative number upon error (e.g. blank message, failed to read to body text), which means *no* message header was written to the messages.dat file, so don't increment the record (block) counter used to create the index (NDX) file.

If msgtoqwk() returns 0, that means that a header was written, but no body text (e.g. a vote message) and that is different than an error.