args: prevent panicking in 'rg -h | rg'

Previously, we relied on clap to handle printing either an error
message, or --help/--version output, in addition to setting the exit
status code. Unfortunately, for --help/--version output, clap was
panicking if the write failed, which can happen in fairly common
scenarios via a broken pipe error. e.g., `rg -h | head`.

We fix this by using clap's "safe" API and doing the printing ourselves.
We also set the exit code to `2` when an invalid command has been given.

Fixes #1125 and partially addresses #1159

tests/regression.rs

@@ -592,6 +592,11 @@ rgtest!(r1130, |dir: Dir, mut cmd: TestCommand| {
     );
 });
 
+// See: https://github.com/BurntSushi/ripgrep/issues/1159
+rgtest!(r1159, |_: Dir, mut cmd: TestCommand| {
+    cmd.arg("--wat").assert_exit_code(2);
+});
+
 // See: https://github.com/BurntSushi/ripgrep/issues/1163
 rgtest!(r1163, |dir: Dir, mut cmd: TestCommand| {
     dir.create("bom.txt", "\u{FEFF}test123\ntest123");