exit: tweak exit status logic

This changes how ripgrep emit exit status codes. In particular, any error
that occurs while searching will now cause ripgrep to emit a `2` exit
code, where as it previously would emit either a `0` or a `1` code based
on whether it matched or not. That is, ripgrep would only emit a `2` exit
code for a catastrophic error.

This tweak includes additional logic that GNU grep adheres to, which seems
like good sense. Namely, if -q/--quiet is given, and an error occurs and
a match occurs, then ripgrep will emit a `0` exit code.

Closes #1159

src/args.rs

@@ -268,6 +268,11 @@ impl Args {
         Ok(builder.build(wtr))
     }
 
+    /// Returns true if and only if ripgrep should be "quiet."
+    pub fn quiet(&self) -> bool {
+        self.matches().is_present("quiet")
+    }
+
     /// Returns true if and only if the search should quit after finding the
     /// first match.
     pub fn quit_after_match(&self) -> Result<bool> {