Better AT commands error handling

[cbosdo]

The sim800 module has a poor error handling. Some are still missing, like:

• AT, AT+CLTS=, AT+CNMI=, AT&W can return ERROR too
• AT+CSCS=, AT+CCLK? can return +CME ERROR: too
• AT+CMGL=ALL can return +CMS ERROR: too
• AT+CMGD= can return ERROR or +CMS ERROR: too