Report "Unexpected token" rather than "Illegal token" since this
error fires for certain valid C/C++ tokens when used in an invalid way
(as well as `@` and `$`, but it's reasonable to report these as
"unexpected" too).
Also we now exit after this error rather than trying to continue
parsing, which used to just lead to a potentially confusing second
error.
For example instead of:
c_unexpected_token.i:1: Error: Illegal token '/='.
c_unexpected_token.i:1: Error: Syntax error - possibly a missing semicolon (';').
we now report:
c_unexpected_token.i:1: Error: Unexpected token '/='.
Olly Betts