Add last error code to PJSUA-LIB account info structure

[bennylp]

Currently, the last registration error status is represented in status field of pjsua_acc_info structure. The behavior is inconsistent, as sometimes this field contains a SIP status code, while at other times it contains pj_status_t error code. For example, when authentication fails, this filed would contain a pj_status_t value relating to authentication failure instead of the 401/407 code.

Also when the status field contains a SIP status code, it is sometimes important to know what the error code that causes the failure. For example, when sending a request fails with SIP status 503, it could be caused by resolver failure (e.g. PJLIB_UTIL_EDNSNOANSWERREC error), which in this cause it would be nice to know this information.

This ticket does the following:

• modifies the code to consistently put SIP status code in the status field.
• adds a new field, last_err, and put any error code here.
• clarifies this in the doxygen documentation/comment.