system([...]): Set v:shell_error=-1 if not executable.

Do _not_ set v:shell_error on parameter validation error.

system([...]) does not invoke a shell, so this change is somewhat
questionable. But `:help v:shell_error` is sufficiently vague to allow
-1 in this case.

src/nvim/eval.c

@@ -16994,8 +16994,12 @@ static void get_system_output_as_rettv(typval_T *argvars, typval_T *rettv,
   }
 
   // get shell command to execute
-  char **argv = tv_to_argv(&argvars[0], NULL, NULL);
+  bool executable = true;
+  char **argv = tv_to_argv(&argvars[0], NULL, &executable);
   if (!argv) {
+    if (!executable) {
+      set_vim_var_nr(VV_SHELL_ERROR, (long)-1);
+    }
     xfree(input);
     return;  // Already did emsg.
   }