Cannot use an error binding pattern in the on-fail-clause

[MaryamZi]

Description:
The spec defines on-fail-clause as

on-fail-clause := on fail typed-binding-pattern statement-block

But only capture binding patterns seem to be allowed atm.

Steps to reproduce:

public function main() {
    do {
        fail error("error!");
    } on fail var error(m) {

    }
}

results in compilation errors.

$ bal run foo.bal 

Compiling source
        foo.bal
ERROR [foo.bal:(4:19,4:19)] missing identifier
ERROR [foo.bal:(4:19,4:19)] missing open brace token
ERROR [foo.bal:(4:25,4:26)] undefined symbol 'm'
ERROR [foo.bal:(4:28,6:6)] incompatible types: expected 'error', found 'record {| |} & readonly'
ERROR [foo.bal:(4:28,4:28)] missing equal token
ERROR [foo.bal:(7:1,7:1)] missing semicolon token
ERROR [foo.bal:(7:2,7:2)] missing close brace token

Affected Versions:
slalpha2, slalpha3-SNAPSHOT

[MaryamZi]

Note that since the typed binding pattern was made optional, this is now

on-fail-clause := on fail [typed-binding-pattern] statement-block