Do not ignore non-existent BAT_CONFIG_PATH

Do not ignore `BAT_CONFIG_PATH` if it doesn't exist. Both when
generating a new config file with `--generate-config-file` and
when attempting to read the config.

Also, provide a better error message in case the file can not
be created.

closes #1550

tests/integration_tests.rs

@@ -652,6 +652,13 @@ fn config_location_test() {
         .assert()
         .success()
         .stdout("bat.conf\n");
+
+    bat_with_config()
+        .env("BAT_CONFIG_PATH", "not-existing.conf")
+        .arg("--config-file")
+        .assert()
+        .success()
+        .stdout("not-existing.conf\n");
 }
 
 #[test]