sd-bus: add custom return code when $XDG_RUNTIME_DIR is not set

We would return ENOENT, which is extremely confusing. Strace is not helpful because
no *file* is actually missing. So let's add some logs at debug level and also use
a custom return code. Let all user-facing utilities print a custom error message
in that case.

src/libsystemd/sd-bus/sd-bus.c

@@ -1340,7 +1340,8 @@ int bus_set_address_user(sd_bus *b) {
 
                 e = secure_getenv("XDG_RUNTIME_DIR");
                 if (!e)
-                        return -ENOENT;
+                        return log_debug_errno(SYNTHETIC_ERRNO(ENOMEDIUM),
+                                               "sd-bus: $XDG_RUNTIME_DIR not set, cannot connect to user bus.");
 
                 ee = bus_address_escape(e);
                 if (!ee)

src/libsystemd/sd-bus/test-bus-cleanup.c

@@ -23,7 +23,7 @@ static int test_bus_open(void) {
         int r;
 
         r = sd_bus_open_user(&bus);
-        if (IN_SET(r, -ECONNREFUSED, -ENOENT)) {
+        if (IN_SET(r, -ECONNREFUSED, -ENOENT, -ENOMEDIUM)) {
                 r = sd_bus_open_system(&bus);
                 if (IN_SET(r, -ECONNREFUSED, -ENOENT))
                         return r;

src/libsystemd/sd-bus/test-bus-track.c

@@ -55,7 +55,7 @@ int main(int argc, char *argv[]) {
         assert_se(r >= 0);
 
         r = sd_bus_open_user(&a);
-        if (IN_SET(r, -ECONNREFUSED, -ENOENT)) {
+        if (IN_SET(r, -ECONNREFUSED, -ENOENT, -ENOMEDIUM)) {
                 r = sd_bus_open_system(&a);
                 if (IN_SET(r, -ECONNREFUSED, -ENOENT))
                         return log_tests_skipped("Failed to connect to bus");