tree-wide: make use of log_error_errno() return value

Turns this:

        r = -errno;
        log_error_errno(errno, "foo");

into this:

        r = log_error_errno(errno, "foo");

and this:

        r = log_error_errno(errno, "foo");
        return r;

into this:

        return log_error_errno(errno, "foo");

src/core/execute.c

@@ -928,8 +928,7 @@ fail:
                 log_error("PAM failed: %s", pam_strerror(handle, pam_code));
                 err = -EPERM;  /* PAM errors do not map to errno */
         } else {
-                log_error_errno(errno, "PAM failed: %m");
-                err = -errno;
+                err = log_error_errno(errno, "PAM failed: %m");
         }
 
         if (handle) {