Improve incomplete unicode escape reporting

This improves diagnostic messages when \u escape is used incorrectly and { is
missing. Instead of saying “unknown character escape: u”, it will now report
that unicode escape sequence is incomplete and suggest what the correct syntax
is.

src/test/parse-fail/issue-23620-invalid-escapes.rs

@@ -41,5 +41,5 @@ fn main() {
     //~^ ERROR illegal unicode character escape
     //~^^ ERROR illegal character in numeric character escape:
     //~^^^ ERROR form of character escape may only be used with characters in the range [\x00-\x7f]
-    //~^^^^ ERROR unknown character escape: u
+    //~^^^^ ERROR incomplete unicode escape sequence
 }