core: improve messages about unknown users and groups

$ sudo build/systemd-run --uid=asdf whoami
$ journalctl -e
(whoami)[1007784]: run-p1007782-i5200512.service: Failed to determine user credentials: No such process
(whoami)[1007784]: run-p1007782-i5200512.service: Failed at step USER spawning /usr/sbin/whoami: No such process
systemd[1]: run-p1007782-i5200512.service: Main process exited, code=exited, status=217/USER
systemd[1]: run-p1007782-i5200512.service: Failed with result 'exit-code'.

Now:
(whoami)[1013204]: run-p1013202-i5205932.service: Failed to determine credentials for user 'asdf': Unknown user
(whoami)[1013204]: run-p1013202-i5205932.service: Failed at step USER spawning /usr/sbin/whoami: Invalid argument
systemd[1]: run-p1013202-i5205932.service: Main process exited, code=exited, status=217/USER
systemd[1]: run-p1013202-i5205932.service: Failed with result 'exit-code'.

src/basic/user-util.h

@@ -9,6 +9,7 @@
 #include <shadow.h>
 
 #include "basic-forward.h"
+#include "errno-util.h"
 
 /* Users managed by systemd-homed. See https://systemd.io/UIDS-GIDS for details
  * how this range fits into the rest of the world. */
@@ -25,6 +26,13 @@
 #define STRERROR_USER(errnum) ((errnum) == -ESRCH ? "Unknown user" : (errnum) == -ENOEXEC ? "Not a system user" : STRERROR(errnum))
 #define STRERROR_GROUP(errnum) ((errnum) == -ESRCH ? "Unknown group" : (errnum) == -ENOEXEC ? "Not a system group" : STRERROR(errnum))
 
+static inline bool ERRNO_IS_NEG_BAD_ACCOUNT(intmax_t r) {
+        return IN_SET(r,
+                      -ESRCH,
+                      -ENOEXEC);
+}
+_DEFINE_ABS_WRAPPER(BAD_ACCOUNT);
+
 bool uid_is_valid(uid_t uid) _const_;
 
 static inline bool gid_is_valid(gid_t gid) {