Better parser recovery techniques

Compiling the attached file:

I think the error message is not useful. Surely a better message is possible.

../bin/x10c x.x10
/Users/sjfink/work/x10-trunk/x10.dist/samples/x.x10:12: Syntax error: Unexpected input ignored: "ERROR_TOKEN"
def this(byte c, double p) {
^

/Users/sjfink/work/x10-trunk/x10.dist/samples/x.x10:12: Syntax error: Unexpected input ignored: "ERROR_TOKEN"
def this(byte c, double p) {
^{--------------}

/Users/sjfink/work/x10-trunk/x10.dist/samples/x.x10:17: Syntax error: Unexpected input ignored: "ERROR_TOKEN"
}