shared/install: avoid prematurely rejecting "missing" units

f7101b7368 copied some logic to prevent enabling masked units, but
also added a check which causes attempts to enable templated units to
fail. Since we know the logic beyond this check will properly handle
units which truly do not exist, we can rely on the unit file state
comparison to suffice for expressing the intent of f7101b7368.

ref: https://bugs.archlinux.org/task/42616
(cherry picked from commit 0ffce503cd)

src/shared/install.c

@@ -1620,12 +1620,10 @@ int unit_file_enable(
         STRV_FOREACH(i, files) {
                 UnitFileState state;
 
+                /* We only want to know if this unit is masked, so we ignore
+                 * errors from unit_file_get_state, deferring other checks.
+                 * This allows templated units to be enabled on the fly. */
                 state = unit_file_get_state(scope, root_dir, *i);
-                if (state < 0) {
-                        log_error("Failed to get unit file state for %s: %s", *i, strerror(-state));
-                        return state;
-                }
-
                 if (state == UNIT_FILE_MASKED || state == UNIT_FILE_MASKED_RUNTIME) {
                         log_error("Failed to enable unit: Unit %s is masked", *i);
                         return -ENOTSUP;