Add ptau validation before using it in groth16 setup

[flipchan]

Fixes a bug that runs setup on invalid ptau file that runs on 100% cpu forever, issue: #622
also fixes:
#614

command i ran for 12h+:

ps waux | grep snarkjs                    
root     16466  100  0.5 11155740 92068 pts/161 Rl+ Apr15 741:48 node /usr/local/bin/snarkjs groth16 setup /tmp/erc20_output/withdraw_phase2_erc20.r1cs /tmp/erc20_output/pot19_final.ptau /tmp/erc20_output/withdraw_phase2_erc20_0000.zkey  ```

[flipchan]

ping @jbaylina plz merge