sleep: Fix whitespace issues. Don't panic when adding durations.

Refactor error propagation and use show_error! instead of USimpleError.

tests/by-util/test_sleep.rs

@@ -1,3 +1,5 @@
+use rstest::rstest;
+
 // spell-checker:ignore dont
 use crate::common::util::*;
 
@@ -180,6 +182,47 @@ fn test_sleep_when_multiple_inputs_exceed_max_duration_then_no_error() {
         .no_output();
 }
 
+#[rstest]
+#[case::whitespace_prefix(" 0.1s")]
+#[case::multiple_whitespace_prefix("   0.1s")]
+#[case::whitespace_suffix("0.1s ")]
+#[case::mixed_newlines_spaces_tabs("\n\t0.1s \n ")]
+fn test_sleep_when_input_has_whitespace_then_no_error(#[case] input: &str) {
+    new_ucmd!()
+        .arg(input)
+        .timeout(Duration::from_secs(10))
+        .succeeds()
+        .no_output();
+}
+
+#[rstest]
+#[case::only_space(" ")]
+#[case::only_tab("\t")]
+#[case::only_newline("\n")]
+fn test_sleep_when_input_has_only_whitespace_then_error(#[case] input: &str) {
+    new_ucmd!()
+        .arg(input)
+        .timeout(Duration::from_secs(10))
+        .fails()
+        .usage_error(format!("invalid time interval '{input}'"));
+}
+
+#[test]
+fn test_sleep_when_multiple_input_some_with_error_then_shows_all_errors() {
+    let expected = "invalid time interval 'abc'\n\
+        sleep: invalid time interval '1years'\n\
+        sleep: invalid time interval ' '";
+
+    // Even if one of the arguments is valid, but the rest isn't, we should still fail and exit early.
+    // So, the timeout of 10 seconds ensures we haven't executed `thread::sleep` with the only valid
+    // interval of `100000.0` seconds.
+    new_ucmd!()
+        .args(&["abc", "100000.0", "1years", " "])
+        .timeout(Duration::from_secs(10))
+        .fails()
+        .usage_error(expected);
+}
+
 #[test]
 fn test_negative_interval() {
     new_ucmd!()