[API] SRT_ECONNSOCK error message does not mention LISTENING and CONNECTING states

[maxsharabayko]

A follow-up of a discussion in #3062:
Trying to set the listener callback on a socket that is in the listening state results in an error SRT_ECONNSOCK (MJ_NOTSUP + MN_ISCONNECTED). The textual representation of this error is "Cannot do this operation on a CONNECTED socket".

In this case, the error message is misleading. The socket is not in a connected state but in a listening state. The error message must be broader or separate errors must be defined for "CONNECTING" and "LISTENING" states. The latter requires API change.