fix https://github.com/lesspass/lesspass/issues/285

Fixes https://github.com/lesspass/lesspass/issues/285

Fixes: https://github.com/lesspass/lesspass/issues/279

see https://github.com/lesspass/cli/pull/7

[CLI] Exit cleanly on cancellation

If I cancel with `Ctrl+C` when `lesspass` prompts me for the master
password, I get the following error:

    $ lesspass example.com test
    master password: TypeError: Pass phrase must be a buffer
        at pbkdf2 (crypto.js:643:20)
        at Object.exports.pbkdf2 (crypto.js:623:10)
        at /usr/local/lib/node_modules/lesspass-cli/node_modules/lesspass/src/pbkdf2.js:40:16
        at Promise (<anonymous>)
        at pbkdf2Browserified (/usr/local/lib/node_modules/lesspass-cli/node_modules/lesspass/src/pbkdf2.js:39:12)
        at calcEntropy (/usr/local/lib/node_modules/lesspass-cli/node_modules/lesspass/src/v2.js:23:12)
        at Object.generatePassword (/usr/local/lib/node_modules/lesspass-cli/node_modules/lesspass/src/v2.js:16:12)
        at Object.generatePassword (/usr/local/lib/node_modules/lesspass-cli/node_modules/lesspass/src/lesspass.js:61:15)
        at calcPassword (/usr/local/lib/node_modules/lesspass-cli/cli.js:51:12)
        at /usr/local/lib/node_modules/lesspass-cli/cli.js:122:5

This is a small fixe to catch the error and exit normally.