Only generate `.json` on successful compile

[lethalgem]

Closes #704

Will no longer generate a -abi.json unless the contract can be compiled. This means any compilation error during parsing, flattening, and asm-gen will prevent the file from being generated/modified

ex.

--parse error--
predicate test {
    type MyMap = (int => int);
}

--flatten error--
predicate test {
    var x = 2;
    var y: ( int => int );
}

--asm-gen error--
predicate test {
    state x = __storage_get(123);
}