[ESP8266] Fixes AT-layer socket_open-functions
[ESP8266] Fixes AT-layer socket_open-functions

When a socket open fails it musn't be treated as fatal error. Returning
a device error which is then handled by an application is the correct
solution

andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed