Add integration test for unparsable pager

Right now all tests pass even if we e.g. return

    Ok(OutputType::stdout())

instead of doing

    .chain_err(|| "Could not parse pager command.")?

so add a test to make sure this functionality don't break.

tests/integration_tests.rs

@@ -522,6 +522,17 @@ fn alias_pager_disable_long_overrides_short() {
         .stdout(predicate::eq("pager-output\n").normalize());
 }
 
+#[test]
+fn pager_failed_to_parse() {
+    bat()
+        .env("BAT_PAGER", "mismatched-quotes 'a")
+        .arg("--paging=always")
+        .arg("test.txt")
+        .assert()
+        .failure()
+        .stderr(predicate::str::contains("Could not parse pager command"));
+}
+
 #[test]
 fn config_location_test() {
     bat_with_config()