Tweak chr out of range error output

* This is nothing to do with UTF-8 at all, so don't mention it
* Bring it in line with the error MoarVM now gives if the codepoint is
  out of range, so we don't get an inconsistency when crossing the "big
  int to small int but still too big" line

src/core.c/Int.pm6

@@ -83,7 +83,7 @@ my class Int does Real { # declared in BOOTSTRAP
     method chr(Int:D: --> Str:D) {
         nqp::if(
           nqp::isbig_I(self),
-            die("Error encoding UTF-8 string: could not encode codepoint %i (0x%X), codepoint out of bounds.".sprintf(self, self)),
+          die("chr codepoint %i (0x%X) is out of bounds".sprintf(self, self)),
           nqp::p6box_s(nqp::chr(nqp::unbox_i(self)))
         )
     }