Fix minor warning w/Ubuntu18.04 in host test build (#5829)
Fix minor warning w/Ubuntu18.04 in host test build (#5829)

GCC reports the following warning about ignoring the return value of
write() (to the console/UART in this instance).

common/MockUART.cpp: In function ‘void uart_do_write_char(int, char)’:
common/MockUART.cpp:67:8: warning: ignoring return value of ‘ssize_t write(int, const void*, size_t)’, declared with attribute warn_unused_result [-Wunused-result]
   write(uart_nr + 1, &c, 1);
   ~~~~~^~~~~~~~~~~~~~~~~~~~

Add a simple warning printout to STDERR which may not be the same as the
UART stream and could succeed.
esp8266/Arduino,esp8266/Arduino,sticilface/Arduino,Lan-Hekary/Arduino,sticilface/Arduino,Lan-Hekary/Arduino,esp8266/Arduino,Lan-Hekary/Arduino,esp8266/Arduino,sticilface/Arduino,Lan-Hekary/Arduino,sticilface/Arduino,sticilface/Arduino,esp8266/Arduino,Lan-Hekary/Arduino