Fix error handling in qtpool.c and other files

[byannoni]

The error handling in some files doesn't seem to have been fully refactored.

• qtpool.c
  • [x] qtstop() - this procedure should just return a constant QTSUCCESS
  • [x] qtstart_get_e() - the procedure should use QTEINVAL instead of errno
• fq/indexed_array_queue.c
  • [x] fqinitia()
  • [x] fqdestroyia()
  • [x] fqpushia()
  • [x] fqpopia()
  • [x] fqpeekia()