Improve poll(2) handling and log messages

 * Better error handling and log messages inside
   uipc_flush_ch_locked()
 * Add missing error check inside UIPC_Read()
 * Add missing OSI_NO_INTR() wrapper around poll()
   inside function btu_exec_tap_fd_read()

Bug: 28823662
Change-Id: I5f1c720861bea594d53ed6465a5ff327ba352598