numfmt: prohibit --header=0 to align with GNU

Adjust header option handling to prohibit passing a value of 0 to align
with GNU numfmt. Also report header option parse errors as GNU does.

closes #1708

src/uu/numfmt/src/numfmt.rs

@@ -291,11 +291,18 @@ fn parse_options(args: &ArgMatches) -> Result<NumfmtOptions> {
 
     let header = match args.occurrences_of(options::HEADER) {
         0 => Ok(0),
-        _ => args
+        _ => {
-            .value_of(options::HEADER)
+            let value = args.value_of(options::HEADER).unwrap();
-            .unwrap()
+
+            value
                 .parse::<usize>()
-            .map_err(|err| err.to_string()),
+                .map_err(|_| value)
+                .and_then(|n| match n {
+                    0 => Err(value),
+                    _ => Ok(n),
+                })
+                .map_err(|value| format!("invalid header value ‘{}’", value))
+        }
     }?;
 
     Ok(NumfmtOptions {
@@ -423,17 +430,16 @@ pub fn uumain(args: impl uucore::Args) -> i32 {
         .arg(Arg::with_name(options::NUMBER).hidden(true).multiple(true))
         .get_matches_from(args);
 
-    let options = parse_options(&matches).unwrap();
+    let result =
-
+        parse_options(&matches).and_then(|options| match matches.values_of(options::NUMBER) {
-    let result = match matches.values_of(options::NUMBER) {
             Some(values) => handle_args(values, options),
             None => handle_stdin(options),
-    };
+        });
 
     match result {
         Err(e) => {
             show_info!("{}", e);
-            exit!(1);
+            1
         }
         _ => 0,
     }

tests/by-util/test_numfmt.rs

@@ -107,6 +107,30 @@ fn test_header_default() {
         .stdout_is("header\n1000\n1100000\n100000000\n");
 }
 
+#[test]
+fn test_header_error_if_non_numeric() {
+    new_ucmd!()
+        .args(&["--header=two"])
+        .run()
+        .stderr_is("numfmt: invalid header value ‘two’");
+}
+
+#[test]
+fn test_header_error_if_0() {
+    new_ucmd!()
+        .args(&["--header=0"])
+        .run()
+        .stderr_is("numfmt: invalid header value ‘0’");
+}
+
+#[test]
+fn test_header_error_if_negative() {
+    new_ucmd!()
+        .args(&["--header=-3"])
+        .run()
+        .stderr_is("numfmt: invalid header value ‘-3’");
+}
+
 #[test]
 fn test_negative() {
     new_ucmd!()