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

Cargo.toml

@@ -23,7 +23,8 @@ default-features = false
 features = ["suggestions", "color", "wrap_help"]
 
 [dev-dependencies]
-assert_cmd = "1.0.1"
+assert_cmd = "1.0"
+predicates = "1.0"
 
 [profile.release]
 lto = true