add TUSB_CFG_OS_TASK_PRIO to mandatory option for using an RTOS

using plain char for error enum character
increase freeRTOS configMAX_PRIORITIES to 16
house keeping & clean up compiler warning

tinyusb/tusb_option.h

@@ -114,6 +114,10 @@
   #ifndef TUSB_CFG_OS_TICKS_PER_SECOND
     #error TUSB_CFG_OS_TICKS_PER_SECOND is required to use with OS_NONE
   #endif
+#else
+  #ifndef TUSB_CFG_OS_TASK_PRIO
+    #error TUSB_CFG_OS_TASK_PRIO need to be defined (hint: use the highest if possible)
+  #endif
 #endif
 
 #ifndef TUSB_CFG_CONFIGURATION_MAX