Make non-ASCII errors more consistent.

There are three kinds of "byte" literals: byte literals, byte string
literals, and raw byte string literals. None are allowed to have
non-ASCII chars in them.

Two `EscapeError` variants exist for when that constraint is violated.
- `NonAsciiCharInByte`: used for byte literals and byte string literals.
- `NonAsciiCharInByteString`: used for raw byte string literals.

As a result, the messages for raw byte string literals use different
wording, without good reason. Also, byte string literals are incorrectly
described as "byte constants" in some error messages.

This commit eliminates `NonAsciiCharInByteString` so the three cases are
handled similarly, and described correctly. The `mode` is enough to
distinguish them.

Note: Some existing error messages mention "byte constants" and some
mention "byte literals". I went with the latter here, because it's a
more correct name, as used by the Reference.

src/test/ui/parser/byte-string-literals.rs

@@ -3,7 +3,7 @@ static FOO: &'static [u8] = b"\f";  //~ ERROR unknown byte escape
 pub fn main() {
     b"\f";  //~ ERROR unknown byte escape
     b"\x0Z";  //~ ERROR invalid character in numeric character escape: `Z`
-    b"é";  //~ ERROR non-ASCII character in byte constant
-    br##"é"##;  //~ ERROR raw byte string must be ASCII
+    b"é";  //~ ERROR non-ASCII character in byte string literal
+    br##"é"##;  //~ ERROR non-ASCII character in raw byte string literal
     b"a  //~ ERROR unterminated double quote byte string
 }