core: introduce a new load state "bad-setting"

Since bb28e68477 parsing failures of
certain unit file settings will result in load failures of units. This
introduces a new load state "bad-setting" that is entered in precisely
this case.

With this addition error messages on bad settings should be a lot more
explicit, as we don't have to show some generic "errno" error in that
case, but can explicitly say that a bad setting is at fault.

Internally this unit load state is entered as soon as any configuration
loader call returns ENOEXEC. Hence: config parser calls should return
ENOEXEC now for such essential unit file settings. Turns out, they
generally already do.

Fixes: #9107

src/basic/unit-def.c

@@ -93,6 +93,7 @@ static const char* const unit_load_state_table[_UNIT_LOAD_STATE_MAX] = {
         [UNIT_STUB] = "stub",
         [UNIT_LOADED] = "loaded",
         [UNIT_NOT_FOUND] = "not-found",
+        [UNIT_BAD_SETTING] = "bad-setting",
         [UNIT_ERROR] = "error",
         [UNIT_MERGED] = "merged",
         [UNIT_MASKED] = "masked"

src/basic/unit-def.h

@@ -30,8 +30,9 @@ typedef enum UnitType {
 typedef enum UnitLoadState {
         UNIT_STUB = 0,
         UNIT_LOADED,
-        UNIT_NOT_FOUND,
-        UNIT_ERROR,
+        UNIT_NOT_FOUND,    /* error condition #1: unit file not found */
+        UNIT_BAD_SETTING,  /* error condition #2: we couldn't parse some essential unit file setting */
+        UNIT_ERROR,        /* error condition #3: other "system" error, catchall for the rest */
         UNIT_MERGED,
         UNIT_MASKED,
         _UNIT_LOAD_STATE_MAX,