Better error message when seek fails

> hexyl "$(which hexyl)" --skip=-5gib
    Error: Failed to jump to the desired input position. This could be
    caused by a negative offset that is too large or by an input that is not
    seek-able (e.g. if the input comes from a pipe).

ref: #114

tests/integration_tests.rs

@@ -84,6 +84,27 @@ fn prints_warning_when_skipping_past_the_end() {
         );
 }
 
+#[test]
+fn can_handle_negative_skip_arguments() {
+    hexyl()
+        .arg("hello_world_elf64")
+        .arg("--color=never")
+        .arg("--skip=-1KiB")
+        .assert()
+        .success();
+}
+
+#[test]
+fn fails_if_negative_offset_is_too_large() {
+    hexyl()
+        .arg("hello_world_elf64")
+        .arg("--color=never")
+        .arg("--skip=-1MiB")
+        .assert()
+        .failure()
+        .stderr(predicates::str::contains("Failed to jump"));
+}
+
 #[test]
 fn fail_if_length_and_bytes_options_are_used_simultaneously() {
     hexyl()