Feature: Log: improve IPC connection error messages

crm_ipc_connect() no longer logs any error messages, instead letting the
callers do it. This allows messages to be logged with the appropriate severity
(instead of always "info" as before) or not at all when appropriate (e.g.
when testing whether an existing daemon is running before starting a new one).
This revision also makes the error messages more user-friendly, adds
NULL tests for a couple of pointers before they are used, and ensures
mainloop_add_fd() always sets errno if it returns NULL.